arch-arm: Initialized some variables

arch-arm: Initialized some variables

Some of the variables in pauth_helpers.cc
are uninitialized in certain control paths
which causes a compiler warning. We initialize
these to false since they should be updated
to the correct value in all valid code paths.

Change-Id: If34d7daaf2404c2cf014c7b4c0c2f979580f36b9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31094
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>