[PowerPC] swdiv builtins for XL compatibility

This patch is in a series of patches to provide builtins for compatibility with
the XL compiler. This patch implements the software divide builtin as
wrappers for a floating point divide. XL provided these builtins because it
didn't produce software estimates by default at `-Ofast`. When compiled
with `-Ofast` these builtins will produce the software estimate for divide.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D106959