x86: fix section mismatch warning in setup_64.c

Fix the following warning:
WARNING: arch/x86/kernel/built-in.o(.cpuinit.text+0x7a3): Section mismatch: reference to .init.text:amd_detect_cmp in 'init_amd'

The function amd_detect_cmp were annotated __init and
was only used from init_amd() which are annotated __cpuinit.

Annotate amd_detect_cmp() with _cpuinit to fix it.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>