[Clang] Mutate long-double math builtins into f128 under IEEE-quad

Under -mabi=ieeelongdouble on PowerPC, IEEE-quad floating point semantic
is used for long double. This patch mutates call to related builtins
into f128 version on PowerPC. And in theory, this should be applied to
other targets when their backend supports IEEE 128-bit style libcalls.

GCC already has these mutations except nansl, which is not available on
PowerPC along with other variants (nans, nansf).

Reviewed By: RKSimon, nemanjai

Differential Revision: https://reviews.llvm.org/D92080