arm64: Call ARCH_WORKAROUND_2 on transitions between EL0 and EL1

In order for the kernel to protect itself, let's call the SSBD mitigation
implemented by the higher exception level (either hypervisor or firmware)
on each transition between userspace and kernel.

We must take the PSCI conduit into account in order to target the
right exception level, hence the introduction of a runtime patching
callback.

Change-Id: Ic4cc012d804dd0c68b557ba58e34e1b8031f1ae2
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Julien Grall <julien.grall@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: 8e2906245f1e3b0d027169d9f2e55ce0548cb96e
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[isaacm@codeaurora.org: remove code for alternative_cb, as
 that is not present. Instead, since the psci conduit is
 smc, replace the alternative_cb reference with the smc
 call it would be anyway.]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>