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

There is a single qemu-system-alpha 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: <20260219191955.83815-44-philmd@linaro.org>

target/alpha/meson.build

@@ -2,7 +2,6 @@ alpha_ss = ss.source_set()
 alpha_ss.add(files(
   'cpu.c',
   'fpu_helper.c',
-  'gdbstub.c',
   'helper.c',
   'clk_helper.c',
   'int_helper.c',
@@ -11,11 +10,18 @@ alpha_ss.add(files(
   'vax_helper.c',
 ))
 
-alpha_system_ss = ss.source_set()
-alpha_system_ss.add(files(
+alpha_user_ss = ss.source_set()
+alpha_user_ss.add(files(
+  'gdbstub.c',
+))
+
+alpha_common_system_ss = ss.source_set()
+alpha_common_system_ss.add(files(
+  'gdbstub.c',
   'machine.c',
   'sys_helper.c',
 ))
 
 target_arch += {'alpha': alpha_ss}
-target_common_system_arch += {'alpha': alpha_system_ss}
+target_user_arch += {'alpha': alpha_user_ss}
+target_common_system_arch += {'alpha': alpha_common_system_ss}