[APFloat] Add exp function for APFloat::IEEESsingle using expf implementation from LLVM libc. (#143959)

Discourse RFC:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

- The implementation in LLVM libc is header-only.
- expf implementation in LLVM libc is correctly rounded for all rounding
modes.
- LLVM libc implementation will round to the floating point
environment's rounding mode.
- No cmake build dependency between LLVM and LLVM libc, only requires
LLVM libc source presents in llvm-project/libc folder.

GitOrigin-RevId: 4190d576823c18f45ee0632baee7d798448178ac