docs/system/gdb.rst: Document how to debug multicore machines

docs/system/gdb.rst: Document how to debug multicore machines

Document how multicore machines appear to GDB when debugged
via the debug stub. This is particularly non-intuitive for
the "multiple heterogenous clusters" case, but unfortunately
as far as I know there is no way with the remote protocol
for the stub to tell gdb "I have 2 inferiors, please connect
to both", so the user must set it all up manually.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210325175023.13838-3-peter.maydell@linaro.org>
Message-Id: <20210401102530.12030-10-alex.bennee@linaro.org>