gdbstub: Introduce GDBFeature structure

Before this change, the information from a XML file was stored in an
array that is not descriptive. Introduce a dedicated structure type to
make it easier to understand and to extend with more fields.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230912224107.29669-6-akihiko.odaki@daynix.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231009164104.369749-13-alex.bennee@linaro.org>

stubs/gdbstub.c

@@ -1,6 +1,6 @@
 #include "qemu/osdep.h"
-#include "exec/gdbstub.h"       /* xml_builtin */
+#include "exec/gdbstub.h"       /* gdb_static_features */
 
-const char *const xml_builtin[][2] = {
-  { NULL, NULL }
+const GDBFeature gdb_static_features[] = {
+  { NULL }
 };