TargetParserTest.ARMExtensionFeatures run out of memory on 32-bit (PR42316)

Nothing of these tests made much sense. Loops were iterating too much, and I
also don't think it was actually testing anything. I think we simply want to
check that AEK_SOME_EXT returns "+some_ext".

I've given the AArch64 tests the same treatment as they very similarly didn't
made any sense either.

This fixes PR42316.

Differential Revision: https://reviews.llvm.org/D63569

llvm-svn: 363913