Make rust emitted code compatible with Rev A Playdate hardware

The FPU settings above are `-mfpu=fpv5-sp-d16`, the `-sp-` part means that double precision floating point is unsupported. The [rust docs](https://doc.rust-lang.org/nightly/rustc/platform-support/thumbv7em-none-eabi.html#table-of-supported-cpus-for-thumbv7em-none-eabihf) show that getting this same effect from rustc-emitted code requires this additional target feature.

Without this, I was getting "undefined instruction" errors for f64 instructions on the original Playdate hardware, with this change it seems to be resolved.

src/main.rs

@@ -660,6 +660,7 @@ impl Build {
                 "RUSTFLAGS",
                 [
                     "-Ctarget-cpu=cortex-m7",
+                    "-Ctarget-feature=-fp64", // Rev A hardware seems to not have 64-bit floating point support
                     "-Clink-args=--emit-relocs",
                     "-Crelocation-model=pic",
                     "-Cpanic=abort",