arm: Teach gem5 to recognize the gem5 semihosting immediate values.

arm: Teach gem5 to recognize the gem5 semihosting immediate values.

These give access to the gem5 extension calls, currently only the pseudo
ops.

Change-Id: I60ece82f1f084791971a2de0b54be2f0d9da243e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27246
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>