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