[PowerPC] Support fp128 libcalls

On PowerPC, IEEE 754 quadruple-precision libcall names use "kf" instead of "tf".

In libgcc, libgcc/config/rs6000/float128-sed converts TF names to KF
names. This patch implements its 24 substitution rules.

Reviewed By: hfinkel

Differential Revision: https://reviews.llvm.org/D64282

llvm-svn: 366039