[benchmark] [PowerPC] Rename mftbl to mftb

`mftb` and `mftbl` are equivalent, yet `mftbl` is not recognized on AIX. Therefore, rename `mftbl` to only have `mftb`.

This mirrors https://reviews.llvm.org/D89506 in LLVM monorepo.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D90779
3 files changed