[MC][AArch64] Enable '+v8a' when nothing specified for MCSubtargetInfo

[MC][AArch64] Enable '+v8a' when nothing specified for MCSubtargetInfo

Since D110065, the 'R' profile support is added to LLVM. It turns the
`generic` cpu into the intersection of v8-a and v8-r. However, this
makes some backward compatibility problems. The original patch makes
the clang driver implicitly pass -march=armv8-a when only the triple
is specified. Since it only applies to clang, other tools like
llvm-objdump still faces the backward compatibility problem.

This patch applies the same idea to MC related tools by enabling '+v8a'
feature when nothing is specified (both CPU and FS are empty) for
MCSubtargetInfo creation.

This patch should fix PR53956.

Reviewed by: labrinea

Differential Revision: https://reviews.llvm.org/D124319