gdbstub: Move syscalls.c out of common_ss

Copy to libuser_ss and libsystem_ss.
This file uses semihosting/semihost.h, which has separate
implementations with and without CONFIG_USER_ONLY.

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

gdbstub/meson.build

@@ -7,15 +7,15 @@
 # We build two versions of gdbstub, one for each mode
 libuser_ss.add(files(
   'gdbstub.c',
+  'syscalls.c',
   'user.c'
 ))
 
 libsystem_ss.add(files(
   'gdbstub.c',
+  'syscalls.c',
   'system.c'
 ))
 
-common_ss.add(files('syscalls.c'))
-
 # The user-target is specialised by the guest
 specific_ss.add(when: 'CONFIG_USER_ONLY', if_true: files('user-target.c'))