[AArch64] Add command-line option for SB

SB (Speculative Barrier) is only mandatory from 8.5
onwards but is optional from Armv8.0-A. This patch adds a command
line option to enable SB, as it was previously only possible to
enable by selecting -march=armv8.5-a.

This patch also moves to FeatureSB the old FeatureSpecRestrict.

Reviewers: pbarrio, olista01, t.p.northover, LukeCheeseman	

Differential Revision: https://reviews.llvm.org/D55921

llvm-svn: 350126