KVM: arm64: Correct PSTATE on exception entry

When KVM injects an exception into a guest, it generates the PSTATE
value from scratch, configuring PSTATE.{M[4:0],DAIF}, and setting all
other bits to zero.

This isn't correct, as the architecture specifies that some PSTATE bits
are (conditionally) cleared or set upon an exception, and others are
unchanged from the original context.

This patch adds logic to match the architectural behaviour. To make this
simple to follow/audit/extend, documentation references are provided,
and bits are configured in order of their layout in SPSR_EL2. This
layout can be seen in the diagram on ARM DDI 0487E.a page C5-429.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200108134324.46500-2-mark.rutland@arm.com