arm64: ssbd: Add prctl interface for per-thread mitigation

If running on a system that performs dynamic SSBD mitigation, allow
userspace to request the mitigation for itself. This is implemented
as a prctl call, allowing the mitigation to be enabled or disabled at
will for this particular thread.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>