[SystemZ] Add -march=archX aliases

For compatibility with other compilers on the platform, allow specifying
levels of the z/Architecture instead of model names with -march.  In
particular, the following aliases are now supported:

  -march=arch8   equals  -march=z10
  -march=arch9   equals  -march=z196
  -march=arch10  equals  -march=zEC12
  -march=arch11  equals  -march=z13

This parallels the equivalent (and prerequisite) LLVM change in r285577.

llvm-svn: 285578