Reinforce ARMTargetParser::getCanonicalArchName validation

Before, getCanonicalArchName was relying on parseArch() to validate the arch
name, which was a problem when other methods, that also needed to call it,
were duplicating the steps.

But to dissociate getCanonicalArchName from parseArch, we needed to make
getCanonicalArchName more robust in detecting valid arch names. It's still
not perfect, but will do for the time being, until we merge Triple with
TargetParser into a TargetDescription mega class.

llvm-svn: 238047