i386/x86-64: fix section mismatch

WARNING: arch/x86_64/kernel/built-in.o - Section mismatch: reference to
.init.text:mtrr_bp_init from .text between 'id entify_cpu' (at offset 0x6571)
and 'IRQ0x20_interrupt'

It's because identify_cpu() which is __cpuinit calls mtrr_bp_init() which is
__init(). __cpuinit() expands to nothing if CONFIG_HOTPLUG_CPU=y and so the
call is illegal.

Signed-off-by: Bernhard Walle <bwalle@suse.de>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>