ARMTargetParser: Normalising build attributes

Now that most of the methods in Clang and LLVM that were parsing arch/cpu/fpu
strings are using ARMTargetParser, it's time to make it a bit more conforming
with what the ABI says.

This commit adds some clarification on what build attributes are accepted and
which are "non-standard". It also makes clear that the "defaultCPU" and
"defaultArch" methods were really just build attribute getters.

It also diverges from GCC's behaviour to say that armv2/armv3 are really an
ARMv4 in the build attributes, when the ABI has a clear state for that: Pre-v4.

llvm-svn: 238344