[compile-rt] Reduce #ifdef noise for ptrauth

Create a sanitizer_ptrauth.h header that #includes <ptrauth> when
available and defines just the required macros as "no ops" otherwise.
This should avoid the need for excessive #ifdef'ing.

Follow-up to and discussed in: https://reviews.llvm.org/D79132

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D79540