arm: Optionally enable gem5 extended semihosting calls.

arm: Optionally enable gem5 extended semihosting calls.

ARM's semihosting interface defines call numbers up to 0xff to be
for standardized use, and says that custom calls should go above this
number.

This new mechanism will let the caller decide whether it wants to
enable these extended calls, or if they should be ignored and only
standard calls should be recognized.

Change-Id: I34b01a4439c8a88242971ac486e34d810b054baf
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25947
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>