[Clang] Mutate bulitin names under IEEE128 on PPC64

Glibc 2.32 and newer uses these symbol names to support IEEE-754 128-bit
float. GCC transforms name of these builtins to align with Glibc header
behavior.

Since Clang doesn't have all GCC-compatible builtins implemented, this
patch only mutates the implemented part.

Note nexttoward is a special case (no nexttowardf128) so it's also
handled here.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D112401