gdbstub: tidy away reverse debugging check into function

gdbstub: tidy away reverse debugging check into function

In theory we don't need an actual record/replay to enact reverse
debugging on a purely deterministic system (i.e one with no external
inputs running under icount). Tidy away the logic into a little
function.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210520174303.12310-8-alex.bennee@linaro.org>