x86/boot/64: Put __startup_64() into .head.text

Put __startup_64() and fixup_pointer() into .head.text section to make
sure it's always near startup_64() and always callable.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel test robot <fengguang.wu@intel.com>
Cc: wfg@linux.intel.com
Link: http://lkml.kernel.org/r/20170616113024.ajmif63cmcszry5a@black.fi.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/head64.c

@@ -38,12 +38,14 @@ extern pmd_t early_dynamic_pgts[EARLY_DYNAMIC_PAGE_TABLES][PTRS_PER_PMD];
 static unsigned int __initdata next_early_pgt;
 pmdval_t early_pmd_flags = __PAGE_KERNEL_LARGE & ~(_PAGE_GLOBAL | _PAGE_NX);
 
-static void __init *fixup_pointer(void *ptr, unsigned long physaddr)
+#define __head	__section(.head.text)
+
+static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
 {
 	return ptr - (void *)_text + (void *)physaddr;
 }
 
-void __init __startup_64(unsigned long physaddr)
+void __head __startup_64(unsigned long physaddr)
 {
 	unsigned long load_delta, *p;
 	pgdval_t *pgd;