alpha: Implement simPalCheck within the ISA description.

alpha: Implement simPalCheck within the ISA description.

This doesn't need to be plumbed through generic interfaces. If the
function/instruction got more complex in the future (unlikely since
Alpha doesn't really see development these days), it could be moved to
a helper function defined within Alpha files.

Change-Id: Ib746fad7bb13c5cc9c6ee555c3a46ce686771c12
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18433
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>