tests/guest-debug: better handle gdb crashes

tests/guest-debug: better handle gdb crashes

There are a number of GDB's on various distros which fail fairly hard
when attempting to talk to a cross-arch guest. The previous attempt to
catch this was incorrect as the shell will deliver signals as 128+n.
Fix the detection and while we are it improve the logging we dump into
the test output.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reported-by: Gautam Agrawal <gautamnagrawal@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-26-alex.bennee@linaro.org>