gdbstub: Add support for target-specific stubs

gdbstub: Add support for target-specific stubs

Currently, it's not possible to have stubs specific to a given target,
even though there are GDB features which are target-specific, like, for
instance, memory tagging.

This commit introduces gdb_extend_qsupported_features,
gdb_extend_query_table, and gdb_extend_set_table functions as interfaces
to extend the qSupported string, the query handler table, and the set
handler table, allowing target-specific stub implementations.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240628050850.536447-4-gustavo.romero@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240705084047.857176-33-alex.bennee@linaro.org>