[X86] Use AVX features instead of ABI to init. SimdDefaultAlign.

The ABI string only exists to communicate with TargetCodeGenInfo.
Concretely, since we only used "avx*" ABI strings on x86_64 (as AVX
doesn't affect the i386 ABIs), this meant that, when initializing
SimdDefaultAlign, we would ignore AVX/AVX512 on i386, for no good
reason.

Instead, directly check the features. A similar change for
MaxVectorAlign will follow.

Differential Revision: http://reviews.llvm.org/D12390

llvm-svn: 246228