Support cross compiler from /.cargo/config linker and rustflags

[bxcrs]

my cross compiler is not in

                        "aarch64-unknown-linux-gnu" => Some("aarch64-linux-gnu"),
                        "aarch64-unknown-linux-musl" => Some("/musl_cc/aarch64-linux-musl-cross/bin/aarch64-linux-musl"),
                        "aarch64-unknown-netbsd" => Some("aarch64--netbsd"),
                        "arm-unknown-linux-gnueabi" => Some("arm-linux-gnueabi"),
                        "armv4t-unknown-linux-gnueabi" => Some("arm-linux-gnueabi"),
                        "armv5te-unknown-linux-gnueabi" => Some("arm-linux-gnueabi"),
                        "armv5te-unknown-linux-musleabi" => Some("arm-linux-gnueabi"),
                        "arm-frc-linux-gnueabi" => Some("arm-frc-linux-gnueabi"),
                        "arm-unknown-linux-gnueabihf" => Some("arm-linux-gnueabihf"),
                        "arm-unknown-linux-musleabi" => Some("/musl_cc/arm-linux-musleabi-cross/bin/arm-linux-musleabi"),
                        "arm-unknown-linux-musleabihf" => Some("/musl_cc/arm-linux-musleabihf-cross/bin/arm-linux-musleabihf"),
                        "arm-unknown-netbsd-eabi" => Some("arm--netbsdelf-eabi"),
                        "armv6-unknown-netbsd-eabihf" => Some("armv6--netbsdelf-eabihf"),
                        "armv7-unknown-linux-gnueabihf" => Some("arm-linux-gnueabihf"),
                        "armv7-unknown-linux-musleabihf" => Some("arm-linux-musleabihf"),
                        "armv7neon-unknown-linux-gnueabihf" => Some("arm-linux-gnueabihf"),
                        "armv7neon-unknown-linux-musleabihf" => Some("arm-linux-musleabihf"),
                        "thumbv7-unknown-linux-gnueabihf" => Some("arm-linux-gnueabihf"),
                        "thumbv7-unknown-linux-musleabihf" => Some("arm-linux-musleabihf"),
                        "thumbv7neon-unknown-linux-gnueabihf" => Some("arm-linux-gnueabihf"),
                        "thumbv7neon-unknown-linux-musleabihf" => Some("arm-linux-musleabihf"),
                        "armv7-unknown-netbsd-eabihf" => Some("armv7--netbsdelf-eabihf"),
                        "i586-unknown-linux-musl" => Some("musl"),
                        "i686-pc-windows-gnu" => Some("i686-w64-mingw32"),
                        "i686-uwp-windows-gnu" => Some("i686-w64-mingw32"),
                        "i686-unknown-linux-musl" => Some("/musl_cc/i686-linux-musl-cross/bin/i686-linux-musl"),
                        "i686-unknown-netbsd" => Some("i486--netbsdelf"),
                        "mips-unknown-linux-gnu" => Some("mips-linux-gnu"),
                        "mips-unknown-linux-musl" => Some("/musl_cc/mips-linux-musl-cross/bin/mips-linux-musl"),
                        "mipsel-unknown-linux-musl" => Some("/musl_cc/mipsel-linux-musl-cross/bin/mipsel-linux-musl"),
                        "mips64-unknown-linux-muslabi64" => Some("/musl_cc/mips64-linux-musl-cross/bin/mips64-linux-musl"),
                        "mips64el-unknown-linux-muslabi64" => Some("/musl_cc/mips64el-linux-musl-cross/bin/mips64el-linux-musl"),
                        "mipsel-unknown-linux-gnu" => Some("mipsel-linux-gnu"),
                        "mips64-unknown-linux-gnuabi64" => Some("mips64-linux-gnuabi64"),
                        "mips64el-unknown-linux-gnuabi64" => Some("mips64el-linux-gnuabi64"),
                        "mipsisa32r6-unknown-linux-gnu" => Some("mipsisa32r6-linux-gnu"),
                        "mipsisa32r6el-unknown-linux-gnu" => Some("mipsisa32r6el-linux-gnu"),
                        "mipsisa64r6-unknown-linux-gnuabi64" => Some("mipsisa64r6-linux-gnuabi64"),
                        "mipsisa64r6el-unknown-linux-gnuabi64" => {
                            Some("mipsisa64r6el-linux-gnuabi64")
                        }
                        "powerpc-unknown-linux-gnu" => Some("powerpc-linux-gnu"),
                        "powerpc-unknown-linux-gnuspe" => Some("powerpc-linux-gnuspe"),
                        "powerpc-unknown-netbsd" => Some("powerpc--netbsd"),
                        "powerpc64-unknown-linux-gnu" => Some("powerpc-linux-gnu"),
                        "powerpc64le-unknown-linux-gnu" => Some("powerpc64le-linux-gnu"),
                        "riscv32i-unknown-none-elf" => Some("riscv32-unknown-elf"),
                        "riscv32imac-unknown-none-elf" => Some("riscv32-unknown-elf"),
                        "riscv32imc-unknown-none-elf" => Some("riscv32-unknown-elf"),
                        "riscv64gc-unknown-none-elf" => Some("riscv64-unknown-elf"),
                        "riscv64imac-unknown-none-elf" => Some("riscv64-unknown-elf"),
                        "riscv64gc-unknown-linux-gnu" => Some("riscv64-linux-gnu"),
                        "s390x-unknown-linux-gnu" => Some("s390x-linux-gnu"),
                        "sparc-unknown-linux-gnu" => Some("sparc-linux-gnu"),
                        "sparc64-unknown-linux-gnu" => Some("sparc64-linux-gnu"),
                        "sparc64-unknown-netbsd" => Some("sparc64--netbsd"),
                        "sparcv9-sun-solaris" => Some("sparcv9-sun-solaris"),
                        "armebv7r-none-eabi" => Some("arm-none-eabi"),
                        "armebv7r-none-eabihf" => Some("arm-none-eabi"),
                        "armv7r-none-eabi" => Some("arm-none-eabi"),
                        "armv7r-none-eabihf" => Some("arm-none-eabi"),
                        "thumbv6m-none-eabi" => Some("arm-none-eabi"),
                        "thumbv7em-none-eabi" => Some("arm-none-eabi"),
                        "thumbv7em-none-eabihf" => Some("arm-none-eabi"),
                        "thumbv7m-none-eabi" => Some("arm-none-eabi"),
                        "thumbv8m.base-none-eabi" => Some("arm-none-eabi"),
                        "thumbv8m.main-none-eabi" => Some("arm-none-eabi"),
                        "thumbv8m.main-none-eabihf" => Some("arm-none-eabi"),
                        "x86_64-pc-windows-gnu" => Some("x86_64-w64-mingw32"),
                        "x86_64-uwp-windows-gnu" => Some("x86_64-w64-mingw32"),
                        "x86_64-rumprun-netbsd" => Some("x86_64-rumprun-netbsd"),
                        "x86_64-unknown-linux-musl" => Some("/musl_cc/x86_64-linux-musl-cross/bin/x86_64-linux-musl"),
                        "x86_64-unknown-netbsd" => Some("x86_64--netbsd"),

[alexcrichton]

Thanks for the report, but I believe this is either covered by #82 or can be fixed with a PR, so closing.