[SystemZ] Simplify handling of ISA revisions

This moves determination of the ISA revision from the CPU name to
one single place, removing a bunch of duplicated code.  It also
makes the supported ISA revisions available as feature strings.

No functional change.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307156 91177308-0d34-0410-b5e6-96231b3b80d8