[ARM] Reject invalid -march values

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236845 91177308-0d34-0410-b5e6-96231b3b80d8