This is step 4 of https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079
A differential fuzzer for these functions has also been added. Along the way, a small correction has been done to the normal/subnormal limits of x86 long double values. Reviewed By: lntue Differential Revision: https://reviews.llvm.org/D94109