arm: Expose the constants which select a semihosting operation.

arm: Expose the constants which select a semihosting operation.

Give these constants meaningful names instead of opaque constants only
visible in the .cc file.

Change-Id: Ib88912dae79960f785099c236c337db52a69d563
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25945
Reviewed-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>