Make relibc (except libm) pure Rust

This reduces the compilation time significantly, both by reducing dependencies (cbindgen is no longer a Cargo dependency, just needs to exist in the PATH), and allowing incremental compilation to be re-enabled. The remaining C files are no longer needed, so relibc, apart from libm, can now be built without a C compiler!

(This uses a sed hack to handle ... va_lists currently, until https://github.com/mozilla/cbindgen/pull/894 is merged.)