arch/x86: Move prologue to .init section

For arch/x86 the realmode part has to be located within the same 64
KiB as the reset vector. Some older intel platforms also require 4 KiB
alignment for _start16bit.

To enforce the above, and to separate required parts of .text without
matching *(.text.*) rules in linker scripts, tag the pre-C environment
assembly code with section .init directive.

Description of .init section for ELF:

This section holds executable instructions that contribute to the
process initialization code. When a program starts to run, the
system arranges to execute the code in this section before calling the
main program entry point (called main for C programs).

Change-Id: If32518b1c19d08935727330314904b52a246af3c
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47599
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/bootblock_crt0.S

@@ -10,7 +10,7 @@
 
 #include <cpu/x86/cr.h>
 
-.section .text._start
+.section .init._start, "ax", @progbits
 
 /*
  * Include the old code for reset vector and protected mode entry. That code has

src/arch/x86/gdt_init.S

@@ -1,7 +1,10 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
 .code32
-.section ".text._gdt_", "ax", @progbits
+
+.section .init, "ax", @progbits
+
+.section .init._gdt_, "ax", @progbits
 
 	.globl gdt_init
 gdt_init:
@@ -17,7 +20,7 @@ gdtptr:
 
 #ifdef __x86_64__
 .code64
-.section ".text._gdt64_", "ax", @progbits
+.section .init._gdt64_, "ax", @progbits
 	.globl gdt_init64
 gdt_init64:
 	/* Workaround a bug in the assembler.

src/arch/x86/walkcbfs.S

@@ -21,7 +21,7 @@
 #define CBFS_FILE_STRUCTSIZE (CBFS_FILE_OFFSET + 4)
 
 .code32
-.section .text
+.section .init
 .global walkcbfs_asm
 
 /*