KVM: arm64: Don't access ICC_SRE_EL2 if GICv3 doesn't support v2 compatibility

KVM: arm64: Don't access ICC_SRE_EL2 if GICv3 doesn't support v2 compatibility

We currently access ICC_SRE_EL2 at each load/put on VHE, and on each
entry/exit on nVHE. Both are quite onerous on NV, as this register
always traps.

We do this to make sure the EL1 guest doesn't flip between v2 and v3
behind our back. But all modern implementations have dropped v2,
and this is just overhead.

At the same time, the GICv5 spec has been fixed to allow access to
ICC_SRE_EL2 in legacy mode. Use this opportunity to replace the
GICv5 checks for v2 compat checks, with an ad-hoc static key.

Co-developed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>