arm: Return whether a semihosting call was recognized/handled.

arm: Return whether a semihosting call was recognized/handled.

Otherwise there's no way to determine whether the return value was from
the semihosting mechanism itself, or from one of the calls. There would
also be no way to determine whether a call had actually happened.

Change-Id: Ie2da812172fe2f9c1e2b5be95561863bd12920b1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25949
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>