arch-arm: Adding isa templates for semihosting ops

A new class of Semihosting constructor templates has been added. Their
main purpose is to check if the Exception Generation Instructions (HLT,
SVC) are actually a semihosting command. If that is the case, the
IsMemBarrier flag is raised, so that in the O3 model we perform a
coherent memory access during the semihosting operation.

Change-Id: Ib87fdeb70ee7a930659563230a80cce0e1372c32
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8370
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>