Implement the SSBD (CVE-2018-3639) workaround on arm64

[andrew]

Summary

This calls into the Arm Trusted Firmware to enable and disable the workaround
for the Speculative Store Bypass Disable (SSBD) issue, also known as Spectre
Variant 4.

As this may have a large performance overhead, and how exploitable SSBD is
is unknown we follow the Linux lead of allowing the administrator to select
between always on, always off, or only enabled in the kernel, with the latter
being the default.

PR: 228955

[andrew]

Fix the SMCCC_ARCH_WORKAROUND_2 function ID

[delphij]

Adding kib@ and emaste@. (I think secteam@ do want to pay attention on this one but is probably more of a "FYI" or "Cc" role rather than a reviewer but thanks for the heads up).