[libc] [llvm] [libc] Add missing MPFR-based tests for math functions (PR #218281)

[via llvm-commits]

tht2005 wrote:

> LGTM after a style nit of replacing with new headers : https://llvm.org/docs/CodingStandards.html#file-headers Also can you rebase so we can check if the bazel error is fixed ?

I'm missing the doxygen file header right? There is no test source file include that, so i skipped it.

https://github.com/llvm/llvm-project/pull/218281