Pass `--remap-path-prefix` to C compilers

[TheBlueMatt]

It would be nice to be able to pass the path remapping stuff from rustc to dependent code to avoid non-determinism slipping in via C dependencies. Presumably this would need to check if the C compiler supports -ffile-prefix-map (at least locally, that's just clang, not GCC).

[dot-asm]

It seems to be possible to parse CARGO_ENCODED_RUSTFLAGS environment variable and simply pass on the reassignments.