target/s390x: Build 'gdbstub.c' once for system single binary

There is a single qemu-system-s390x 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: <20260219191955.83815-49-philmd@linaro.org>

target/s390x/meson.build

@@ -3,7 +3,6 @@ s390x_ss.add(files(
   'cpu.c',
   'cpu_features.c',
   'cpu_models.c',
-  'gdbstub.c',
   'interrupt.c',
   'cpu-dump.c',
 ))
@@ -34,11 +33,13 @@ s390x_common_system_ss.add(files(
   'sigp.c',
   'cpu-system.c',
   'cpu_models_system.c',
+  'gdbstub.c',
 ))
 
 s390x_user_ss = ss.source_set()
 s390x_user_ss.add(files(
   'cpu_models_user.c',
+  'gdbstub.c',
 ))
 
 subdir('tcg')