Cull non-standard variants of ARM architectures (NFC)

Summary:
This patch changes ARMV5, ARMV5E, ARMV6SM, ARMV6HL, ARMV7, ARMV7L,
ARMV7HL, ARMV7EM to be treated as aliases for the corresponding
standard architectures, instead of as actual architectures.

Reviewers: rengolin

Subscribers: aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D14577

llvm-svn: 252903