x86: fix section mismatch in head_64.S:initial_code

initial_code are initially used to hold a function pointer
from __init and later from __cpuinit. This confuses modpost
and changing initial_code to REFDATA silence the warning.
(But now we do not discard the variable anymore).
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/head_64.S

@@ -255,7 +255,7 @@ ENTRY(secondary_startup_64)
 	lretq
 
 	/* SMP bootup changes these two */
-	__CPUINITDATA
+	__REFDATA
 	.align	8
 	ENTRY(initial_code)
 	.quad	x86_64_start_kernel