system: Move most files to libsystem_ss

Some of the headers used require CONFIG_USER_ONLY.
Do not move vl.c, because it has other include dependencies
that are present in system_ss.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

system/meson.build

@@ -4,6 +4,10 @@ specific_ss.add(when: 'CONFIG_SYSTEM_ONLY', if_true: [files(
 )])
 
 system_ss.add(files(
+  'vl.c',
+), sdl, libpmem, libdaxctl)
+
+libsystem_ss.add(files(
   'balloon.c',
   'bootdevice.c',
   'cpus.c',
@@ -23,9 +27,8 @@ system_ss.add(files(
   'runstate-hmp-cmds.c',
   'runstate.c',
   'tpm-hmp-cmds.c',
-  'vl.c',
   'watchpoint.c',
-), sdl, libpmem, libdaxctl)
+))
 
 if have_tpm
   system_ss.add(files('tpm.c'))