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.

Change-Id: I94a21f74b17bc0f35ca141efa21356c9d16bd3eb
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>
Git-commit: 9cdc0108baa8ef87c76ed834619886a46bd70cbe
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[isaacm@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>