linux-user: Move thunk.c from top-level

So far, linux-user is the only user of these functions.
Clean up the build machinery by restricting it to linux-user.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/meson.build

@@ -15,6 +15,7 @@ linux_user_ss.add(files(
   'signal.c',
   'strace.c',
   'syscall.c',
+  'thunk.c',
   'uaccess.c',
   'uname.c',
 ))