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/soc/intel/quark/bootblock/esram_init.S

@@ -71,6 +71,8 @@
 .equ  CFGNONSTICKY_W1_OFFSET, (0x52)
 .equ    FORCE_WARM_RESET, (0x00000001)
 
+.section .init, "ax", @progbits
+
 	.global bootblock_pre_c_entry
 
 bootblock_pre_c_entry: