[SystemZ] Rework processor feature definitions and add -mcpu=archX support

This patch implements two changes:

- Move processor feature definition into a new file SystemZFeatures.td,
  and provide explicit lists of supported and unsupported features for
  each level of the z/Architecture.  This allows specifying unsupported
  features in the scheduler definition files for each processor.

- Add optional aliases for the -mcpu processor names according to the
  level of the z/Architecture, for compatibility with other compilers
  on the platform.  The supported aliases are:
    -mcpu=arch8  equals  -mcpu=z10
    -mcpu=arch9  equals  -mcpu=z196
    -mcpu=arch10 equals  -mcpu=zEC12
    -mcpu=arch11 equals  -mcpu=z13

llvm-svn: 285577