[libc][math] Refactor ldexpf implementation to header-only in src/__support/math folder (#147906)
Part of #147386
in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450
Please merge #147901 first
@lntue