[Clang] Enable KF and KC mode for [_Complex] __float128

The headers provided with recent GNU toolchains for PPC have code that includes
typedefs such as:

typedef _Complex float __cfloat128 __attribute__ ((__mode__ (__KC__)))

This patch allows clang to compile programs that contain
#include <math.h>

with -mfloat128 which it currently fails to compile.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=46068

Differential revision: https://reviews.llvm.org/D80374