sim: Use named constants for pseudo ops

sim: Use named constants for pseudo ops

Use named constants from a shared header instead of magic values when
handling pseudo ops.

Change-Id: If157060bbcd772ce7e8556482b44ca714f4319b1
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/4262
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>