[PowerPC] Provide fastmath sqrt and div functions in altivec.h

This adds the long overdue implementations of these functions
that have been part of the ABI document and are now part of
the "Power Vector Intrinsic Programming Reference" (PVIPR).

The approach is to add new builtins and to emit code with
the fast flag regardless of whether fastmath was specified
on the command line.

Differential revision: https://reviews.llvm.org/D101209