arm64: KVM: Add patchable function selector

arm64: KVM: Add patchable function selector

KVM so far relies on code patching, and is likely to use it more
in the future. The main issue is that our alternative system works
at the instruction level, while we'd like to have alternatives at
the function level.

In order to cope with this, add the "hyp_alternate_select" macro that
outputs a brief sequence of code that in turn can be patched, allowing
an alternative function to be selected.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
(cherry picked from commit c1bf6e18e97e7ead77371d4251f8ef1567455584)
Signed-off-by: Alex Shi <alex.shi@linaro.org>