[L] Change in coreboot[main]: libpayload: Add x86_64 (64-bit) support

[Subrata Banik (Code Review)]

Attention is currently required from: Arthur Heymans, Julius Werner, Kapil Porwal.

Hello build bot (Jenkins),

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

https://review.coreboot.org/c/coreboot/+/81968?usp=email

to look at the new patch set (#77).

The following approvals got outdated and were removed: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

Change subject: libpayload: Add x86_64 (64-bit) support
......................................................................

libpayload: Add x86_64 (64-bit) support

This patch introduces x86_64 (64-bit) support to the payload, building
upon the existing x86 (32-bit) architecture. Files necessary for 64-bit
compilation are now guarded by the `CONFIG_LP_ARCH_X86_64` Kconfig
option.

BUG=b:242829490
TEST=Able to verify all valid combinations between coreboot and
payload with this patch.

Payload Entry Point Behavior with below code.

+----------------+--------------------+----------------------------+
| LP_ARCH_X86_64 | Payload Entry Mode | Description                |
+----------------+--------------------+----------------------------+
| No             | 32-bit             | Direct protected mode init |
+----------------+--------------------+----------------------------+
| Yes            | 32-bit             | Protected to long mode     |
+----------------+--------------------+----------------------------+
| Yes            | 64-bit             | Long mode initialization   |
+----------------+--------------------+----------------------------+

Change-Id: I69fda47bedf1a14807b1515c4aed6e3a1d5b8585
Signed-off-by: Subrata Banik 
---
M payloads/libpayload/Kconfig
M payloads/libpayload/Makefile
M payloads/libpayload/Makefile.mk
M payloads/libpayload/arch/x86/Makefile.mk
M payloads/libpayload/arch/x86/exception.c
A payloads/libpayload/arch/x86/exception_asm_64.S
R payloads/libpayload/arch/x86/exec.c
M payloads/libpayload/arch/x86/gdb.c
A payloads/libpayload/arch/x86/head_64.S
M payloads/libpayload/arch/x86/libpayload.ldscript
A payloads/libpayload/arch/x86/pt.S
M payloads/libpayload/arch/x86/string.c
M payloads/libpayload/bin/lpgcc
M payloads/libpayload/drivers/storage/ahci_common.c
M payloads/libpayload/drivers/usb/uhci.c
M payloads/libpayload/include/x86/arch/exception.h
M payloads/libpayload/libc/Makefile.mk
M payloads/libpayload/vboot/Makefile.mk
18 files changed, 686 insertions(+), 58 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/68/81968/77