[PowerPC] Implement builtin for mffsl

mffsl is available since ISA 3.0. The builtin is named with ppc prefix
to follow our convention. For targets earlier than power9, GCC generates
extra code to support the functionality, while this patch does not
implement such behavior.

Reviewed By: nemanjai, tuliom

Differential Revision: https://reviews.llvm.org/D158065