x86: fix asm-x86/byteorder.h for userspace export

Since asm-x86/byteorder.h is exported to userspace, use __asm__ rather than
asm in its code.

Signed-Off-By: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>