Expose cc-rs no_default_flags for hassle-free cross-compilation

[ivmarkov]

In our own instantiation of cmake-rs Config, we would like to suppress the cc-rs logic that generates GCC flags based on Rust target triple analysis. Two reasons for that:

• The current logic in cc-rs is not really working well for (at least some?) RISCV targets
• We anyway have all the necessary flags, and we would like to pass those as-is - no more - no less flags, so there is no real need for the cc-rs crate to help us in that

[ivmarkov]

@tgross35 I'm so sorry for pinging you every now and then (last time in the libc crate). :(

Due to recent hurdles with the cc-rs crate when cross-compiling ESP-IDF, we need this enhancement relatively urgently.

Do you think you could spend a few minutes reviewing it? Hopefully, the change is small enough and would actually benefit everybody, who is in a cross-compiling use case.