[clang-tools-extra] [llvm] [clang] [PowerPC] Implement llvm.set.rounding intrinsic (PR #67302)

[Qiu Chaofan via cfe-commits]

ecnelises wrote:

> Maybe we can do some perf test between this expansion for set rounding mode and the system library's version for fesetround().

They are faster than system `fesetround` on both Linux and AIX. Linux glibc optimizes `fesetround` with faster `mffscrn` on P9, I just exploited the instruction here.

https://github.com/llvm/llvm-project/pull/67302