[AArch64] Add command-line option for SSBS

Summary:
SSBS (Speculative Store Bypass Safe) is only mandatory from 8.5
onwards but is optional from Armv8.0-A. This patch adds testing for
the ssbs command line option, added to allow enabling the feature
in previous Armv8-A architectures to 8.5.

Reviewers: olista01, samparker, aemerson

Reviewed By: samparker

Subscribers: javed.absar, kristof.beyls, cfe-commits

Differential Revision: https://reviews.llvm.org/D54961

llvm-svn: 348142