[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 a command
line option to enable SSBS, as it was previously only possible to
enable by selecting -march=armv8.5-a.

Similar patch upstream in GNU binutils:
https://sourceware.org/ml/binutils/2018-09/msg00274.html

Reviewers: olista01, samparker, aemerson

Reviewed By: samparker

Subscribers: javed.absar, kristof.beyls, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D54629

llvm-svn: 348137