arm64: cpufeature: Detect SSBS and advertise to userspace

Armv8.5 introduces a new PSTATE bit known as Speculative Store Bypass
Safe (SSBS) which can be used as a mitigation against Spectre variant 4.

Additionally, a CPU may provide instructions to manipulate PSTATE.SSBS
directly, so that userspace can toggle the SSBS control without trapping
to the kernel.

This patch probes for the existence of SSBS and advertise the new instructions
to userspace if they exist.

Change-Id: Ieeb4f8d70afcc1d610716f0d1306a73093ff2660
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: d71be2b6c0e19180b5f80a6d42039cc074a693a2
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[neeraju@codeaurora: Resolve trivial merge conflicts]
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>