configure: allow user to specify what gdb to use

This is useful, especially when testing relatively new gdbstub
features that might not be in distro packages yet.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200316172155.971-22-alex.bennee@linaro.org>
1 file changed