target/m68k: Build 'gdbstub.c' once for system single binary

There is a single qemu-system-m68k 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: <20260219191955.83815-47-philmd@linaro.org>

target/m68k/meson.build

@@ -2,13 +2,18 @@ m68k_ss = ss.source_set()
 m68k_ss.add(files(
   'cpu.c',
   'fpu_helper.c',
-  'gdbstub.c',
   'helper.c',
   'op_helper.c',
   'softfloat.c',
   'translate.c',
 ))
 
+m68k_user_ss = ss.source_set()
+m68k_user_ss.add(files('gdbstub.c'))
+
+m68k_common_system_ss = ss.source_set()
+m68k_common_system_ss.add(files('gdbstub.c'))
+
 m68k_system_ss = ss.source_set()
 m68k_system_ss.add(files(
   'monitor.c'
@@ -19,4 +24,6 @@ m68k_system_ss.add(when: ['CONFIG_SEMIHOSTING'],
 )
 
 target_arch += {'m68k': m68k_ss}
+target_user_arch += {'m68k': m68k_user_ss}
 target_system_arch += {'m68k': m68k_system_ss}
+target_common_system_arch += {'m68k': m68k_common_system_ss}