This patch adds bazel tests for llvm-libc.
Some math tests rely on the mpfr library. This is controlled via the --@llvm-project//libc:libc_math_mpfr flag. It can take three values:
- external (default) will build mpfr and gmp from source.
- system will use the system installed mpfr library.
- disable will skip tests relying on mpfr.
It seems like we don't actually use this anywhere?