kvm, arm: Fix incorrect PSTATE sync

kvm, arm: Fix incorrect PSTATE sync

The state transfer code wasn't reading back PSTATE correctly from the
CPU prior to updating the thread context and was incorreclty writing
the register as a 32-bit value when updating KVM. Correctly read back
the state before updating gem5's view of PSTATE and cast the value to
a uint64_t.

Change-Id: I0a6ff5b77b897c756b20a20f65c420f42386360f
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2963
Reviewed-by: Rahul Thakur <rjthakur@google.com>