[llvm] [PowerPC] Remove `UnsafeFPMath` uses (PR #154901)

[via llvm-commits]

RolandF77 wrote:

> and the doc string (sorry I couldn't find the doc about `xsrdpic` in IBM doc site)
> 

PPC ISA should be available here: https://openpowerfoundation.org/specifications/isa/

https://github.com/llvm/llvm-project/pull/154901