KVM: arm64: Stop save/restoring ACTLR_EL1

KVM sets HCR_EL2.TACR (which it calls HCR_TAC) via HCR_GUEST_FLAGS.
This means ACTLR* accesses from the guest are always trapped, and
always return the value in the sys_regs array.

The guest can't change the value of these registers, so we are
save restoring the reset value, which came from the host.

Stop save/restoring this register.

This also stops this register being affected by sysregs_loaded_on_cpu,
so we can provide 32 bit accessors that always use the in-memory copy.

Signed-off-by: James Morse <james.morse@arm.com>