[M] Change in coreboot[main]: arch/x86: Unify GDT entries

[Patrick Rudolph (Code Review)]

Attention is currently required from: Jérémy Compostella, Patrick Rudolph, Shuo Liu.

Hello Jérémy Compostella, Shuo Liu, build bot (Jenkins),

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/87255?usp=email

to look at the new patch set (#3).

The following approvals got outdated and were removed:
Verified-1 by build bot (Jenkins)

Change subject: arch/x86: Unify GDT entries
......................................................................

arch/x86: Unify GDT entries

Currently there are 3 GDTs used on x86:
- preRAM (gdt_init.S)
- SMM (smm_stub.S)
- RAM (c_start.S)

They have different layouts and thus different offset for the segments
being using in assembly code. Stop using different GDT segments and make
sure that for ROM (preRAM + SMM) and RAM (ramstage) the segments match.
RAM will have additional entries, not found in pre RAM GDT, but the segments
for protected mode and 64-bit mode now match in all stages.

This allows to use the same defines in all stages. It also drops the need to
know in which stage the code is compiled and it's no longer necessary to
switch the code segment between stages.

While on it fix the comments in the ramstage GDT and drop unused declarations
from header files.

Change-Id: I208496e6e4cc82833636f4f42503b44b0d702b9e
Signed-off-by: Patrick Rudolph 
---
M src/arch/x86/c_start.S
D src/arch/x86/include/arch/ram_segs.h
D src/arch/x86/include/arch/rom_segs.h
M src/arch/x86/wakeup.S
M src/cpu/x86/64bit/entry64.inc
M src/cpu/x86/64bit/exit32.inc
M src/cpu/x86/entry16.S
M src/cpu/x86/entry32.S
M src/cpu/x86/sipi_vector.S
M src/cpu/x86/smm/smm_stub.S
M src/device/oprom/realmode/x86_asm.S
M src/include/cpu/x86/gdt.h
M src/security/intel/stm/StmPlatformSmm.c
M src/security/intel/txt/getsec_enteraccs.S
14 files changed, 81 insertions(+), 125 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/55/87255/3