[ARM] 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 renames FeatureSpecRestrict to FeatureSB.

Reviewed By: olista01, LukeCheeseman

Differential Revision: https://reviews.llvm.org/D55990

llvm-svn: 350299