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>