[llvm] [libc] [NFC] rename builtin_wrapper into math_extras (PR #72998)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> and bit_support. WDYT? clz functions are actually in "include/llvm/ADT/bit.h"

Is there value in matching the header name from LLVM? i.e. "bit.h" rather than "bit_support.h"?

https://github.com/llvm/llvm-project/pull/72998