gdbstub: Replace GdbCmdContext with plain g_array()

Instead of jumping through hoops let glib deal with both tracking the
number of elements and auto freeing the memory once we are done. This
allows is to drop the usage of ALLOCA(3) which the man-page mentions
its "use is discouraged".

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210520174303.12310-6-alex.bennee@linaro.org>