Lower generic MASSV entries to PowerPC subtarget-specific entries

This patch (second of two patches) lowers the generic PowerPC vector
entries to PowerPC subtarget-specific entries.
For instance, the PowerPC generic entry 'cbrtd2_massv' is lowered to
'cbrtd2_P9' or Power9 subtarget.

The first patch enables the vectorizer to recognize the IBM MASS vector
library routines. This patch specifically adds support for recognizing
the '-vector-library=MASSV' option, and defines mappings from IEEE
standard scalar math functions to generic PowerPC MASS vector
counterparts.
For instance, the generic PowerPC MASS vector entry for double-precision
'cbrt' function is '__cbrtd2_massv'

The overall support for MASS vector library is presented as such in two
patches for ease of review.

Patch by pjeeva01 (Jeeva P.)
Differential Revision: https://reviews.llvm.org/D59883