[PPC] Add support for tune-cpu attribute

clang (like gcc) has the -mtune= command line option. This option
adds the "tune-cpu" attribute to a function. The intended functionality
is that the scheduling model of that cpu is used. E.g. -mtune=pwr9 -march=pwr8
generates only instructions supported on pwr8 but uses the scheduling model
of pwr9 for it.
This PR adds the infrastructure to support this in LLVM.
clang support was added in https://reviews.llvm.org/D130526.

Reviewed By: amyk, qiucf

Differential Revision: https://reviews.llvm.org/D138317