x86: add _ASM_EXTABLE macro to <asm/asm.h>

Instead of open-coding the __ex_table information at each callsite,
construct a common macro that can work regardless of CPU size.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>