x86: use _ASM_EXTABLE macro in arch/x86/mm/init_32.c

Use the _ASM_EXTABLE macro from <asm/asm.h>, instead of open-coding
__ex_table entires in arch/x86/mm/init_32.c.

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>