Fix the following error:

> >   LINK     arch/x86/syscalls.built-in.o
> > arch/x86/crtools.c:36:20: error: unused function '__check_code_syscall'
> >       [-Werror,-Wunused-function]
> > static inline void __check_code_syscall(void)

As the function consists of a few BUILD_BUG_ONs, it gets optimized out.
Let's add __attribute__((__unused__)) so clang stops complaining.

[v2: s/used/unused/, fix all the arches, whitespace cleanup]
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>