arch-arm: Introduce HavePACExt helper

arch-arm: Introduce HavePACExt helper

This will check for presence of pointer authentication extension.
According to the reference manual, Pointer authentication is
implemented if the value of at least one of

ID_AA64ISAR1_EL1.{APA, API, GPA, GPI}

is not 0b0000.

Change-Id: I4e98e65758e8edc953794e5b618d2c6c3f6000ae
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33454
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>