[PowerPC] Add XL Compat fetch builtins

Prototype
```
unsigned int __fetch_and_add (volatile unsigned int* addr, unsigned int
val);
unsigned long __fetch_and_addlp (volatile unsigned long* addr, unsigned
long val);
```
Ref:
https://www.ibm.com/docs/en/xl-c-and-cpp-linux/16.1.1?topic=functions-fetch

Reviewed By: #powerpc, w2yehia, lkail

Differential Revision: https://reviews.llvm.org/D104991