arch-arm: Semihosting not available in syscall emulation

arch-arm: Semihosting not available in syscall emulation

Arm Semihosting is not available in syscall emulation since we don't
have an Arm system in that scenario. Trying to use it in "se" mode will
make getArmSystem assertion fail.

Change-Id: I4cf49ae801ec6e6c93134ac6ae2a0f412040684c
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8367
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>