fastmodel: Enable semihosting, including pseudo insts.

fastmodel: Enable semihosting, including pseudo insts.

It is assumed that the semihosting configuration uses the semihosting
number which includes gem5's pseudo insts.

Given the complexity and likely limitted value of letting the user
arbitrarily configure fast model's semihosting, and the fact that that
semihosting implementation would compete with gem5's own, those
parameters should be removed from python and set purely within C++.

Also note that if this semihosting support is used, the System object
needs to have an ArmSemihosting object installed to handle the calls.

Change-Id: I8e1de7717c9784dc7873795acd0a06389ec527b1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25623
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>