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

[Guillaume Chatelet via llvm-commits]

gchatelet 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"?

We're already in the `__support` folder so I guess "bit.h" would do :) 

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