target/rx: Build 'gdbstub.c' once for system single binary

There is a single qemu-system-rx binary, but by moving
'gdbstub.c' in the target_common_system_arch[] source set
the resulting object can be linked into a single qemu-sytem
binary.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20260219191955.83815-48-philmd@linaro.org>

target/rx/meson.build

@@ -9,8 +9,10 @@ rx_ss.add(files(
   'op_helper.c',
   'helper.c',
   'cpu.c',
-  'gdbstub.c',
   'disas.c'))
 
+rx_common_system_ss = ss.source_set()
+rx_common_system_ss.add(files('gdbstub.c'))
+
 target_arch += {'rx': rx_ss}
-target_common_system_arch += {'rx': ss.source_set()}
+target_common_system_arch += {'rx': rx_common_system_ss}