[reland][libc][bazel] Add tests to the bazel build

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`.

Reviewed By: sivachandra, GMNGeoffrey

Differential Revision: https://reviews.llvm.org/D119547