arm64: Add skeleton to harden the branch predictor against aliasing attacks

Aliasing attacks against CPU branch predictors can allow an attacker to
redirect speculative control flow on some CPUs and potentially divulge
information from one context to another.

This patch adds initial skeleton code behind a new Kconfig option to
enable implementation-specific mitigations against these attacks for
CPUs that are affected.

Change-Id: I778641a6a2658cded427204a8c2b01dc6b6b9579
Co-developed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: 0f15adbb2861ce6f75ccfc5a92b19eae0ef327d0
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux


[neeraju@codeaurora.org: Fix merge conflicts and build failures
 due to missing includes and api changes in new kernels]
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>