[SystemZ] 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=z15 -march=z14`
generates only instructions supported on z14 but uses the scheduling model
of z15 for it.
This PR adds the infrastructure to support this.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D128910