[ARM] Reject invalid -march values

Restructure Triple::getARMCPUForArch so that invalid values will
return nullptr, while retaining the behaviour that an argument
specifying no particular architecture version will give a default
CPU. This will be used by clang to give an error on invalid -march
values.

Also restructure the extraction of the architecture version from
the MArch string a little to hopefully make what it's doing clearer.

Differential Revision: http://reviews.llvm.org/D9599

llvm-svn: 236845