util: Allow overriding the magic address in the m5 utility.

util: Allow overriding the magic address in the m5 utility.

This is useful in situations where the address is hard to know ahead of
time, for instance on ARM systems where the address map is hard to
predict.

The default address is now M5OP_ADDR, or 0 if that's not defined.

Change-Id: I3140e05b04365c1a76e52f8c3dc85f472c230ae4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27244
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>