add relibm crate

[augustin-cheron]

Add a crate to build a c ABI compatible libm in order to test it against other libm test suite.
This crate currently support libc-test use in musl and openlibm tests and benchmark.

[tgross35]

Hey! Sorry this hasn't been looked at in a while. If you are still interested in adding this functionality I think we could do this, but it doesn't need to be a separate crate. Instead, just add a src/no_mangle.rs file that wraps each function in a #[no_mangle] wrapper (use a macro similar to libm_helper rather than enumerating everything manually) and is gated behind a new no-mangle Cargo feature. Running libc-test is also a good idea, that can come as a follow up if it is easier.

Also fyi this PR has a checked in binary.