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

There is a single qemu-system-avr 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: <20260219191955.83815-45-philmd@linaro.org>

target/avr/meson.build

@@ -11,10 +11,13 @@ avr_ss.add(files(
   'translate.c',
   'helper.c',
   'cpu.c',
-  'gdbstub.c',
   'disas.c'))
 
+avr_common_system_ss = ss.source_set()
+avr_common_system_ss.add(files('gdbstub.c'))
+
 avr_system_ss.add(files('machine.c'))
 
 target_arch += {'avr': avr_ss}
 target_system_arch += {'avr': avr_system_ss}
+target_common_system_arch += {'avr': avr_common_system_ss}