gdbstub: Re-factor gdb command extensions

gdbstub: Re-factor gdb command extensions

Coverity reported a memory leak (CID 1549757) in this code and its
admittedly rather clumsy handling of extending the command table.
Instead of handing over a full array of the commands lets use the
lighter weight GPtrArray and simply test for the presence of each
entry as we go. This avoids complications of transferring ownership of
arrays and keeps the final command entries as static entries in the
target code.

Cc: Akihiko Odaki <akihiko.odaki@daynix.com>
Cc: Gustavo Bueno Romero <gustavo.romero@linaro.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Gustavo Romero <gustavo.romero@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240718094523.1198645-4-alex.bennee@linaro.org>