arm64: ssbd: Skip apply_ssbd if not using dynamic mitigation

In order to avoid checking arm64_ssbd_callback_required on each
kernel entry/exit even if no mitigation is required, let's
add yet another alternative that by default jumps over the mitigation,
and that gets nop'ed out if we're doing dynamic mitigation.

Think of it as a poor man's static key...

Change-Id: Idc527c176f171542d22fc43088bd8b84a6a8a790
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
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: 986372c4367f46b34a3c0f6918d7fb95cbdf39d6
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[isaacm@codeaurora.org: Add assembly logic to compare
 ssbd_state value and branch to target label, since the
 alternative_cb macro is not implemented.]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>