meson: Move linux_user_ss to linux-user/

We have no need to reference linux_user_ss outside of linux-user.
Go ahead and merge it directly into specific_ss.

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