Move linux-user safe-syscall.S and safe-syscall-error.c to common-user
so that bsd-user can also use it. Also move safe-syscall.h to
include/user/. Since there is nothing here that is related to the guest,
as opposed to the host, build it once.
Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
