x86/head/64: Move the __head definition to <asm/init.h>

Move the __head section definition to a header to widen its use.

An upcoming patch will mark the code as __head in mem_encrypt_identity.c too.
Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/0583f57977be184689c373fe540cbd7d85ca2047.1697525407.git.houwenlong.hwl@antgroup.com

arch/x86/include/asm/init.h

@@ -2,6 +2,8 @@
 #ifndef _ASM_X86_INIT_H
 #define _ASM_X86_INIT_H
 
+#define __head	__section(".head.text")
+
 struct x86_mapping_info {
 	void *(*alloc_pgt_page)(void *); /* allocate buf for page table */
 	void *context;			 /* context for alloc_pgt_page */

arch/x86/kernel/head64.c

@@ -41,6 +41,7 @@
 #include <asm/trapnr.h>
 #include <asm/sev.h>
 #include <asm/tdx.h>
+#include <asm/init.h>
 
 /*
  * Manage page tables very early on.
@@ -84,8 +85,6 @@ static struct desc_ptr startup_gdt_descr __initdata = {
 	.address = 0,
 };
 
-#define __head	__section(".head.text")
-
 static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
 {
 	return ptr - (void *)_text + (void *)physaddr;