[ARM] Allow TargetParser to accurately target architectures

Instead of defaulting to an empty string, we want to default to
the CPU 'generic' in the case of no valid default CPU being found,
(as long as the architecture is actually valid).

In order to do this we add a default FPU for each architecture, as
well as falling back to architecture defaults for extensions and FPU
in the case of a generic CPU is specified.

llvm-svn: 253198