[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.

Subrata Banik has posted comments on this change by Subrata Banik. ( https://review.coreboot.org/c/coreboot/+/81968?usp=email )

Change subject: libpayload: Add x86_64 (64-bit) support
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Patch Set 69:

(2 comments)

File payloads/libpayload/arch/x86/pt.S:

https://review.coreboot.org/c/coreboot/+/81968/comment/9a56c7de_102d2d02?usp... :
PS64, Line 82: 	incl %ebx
> Can you tell which instruction gives you an exception when you try that?

there were no exception. it was stuck inside the loop looks like. I'm using postcode to debug the hang (w/o any HW debugger hence, unable to tell exact place inside the loop during the hang).

Also, I felt below instruction should be different between 32-bit and 64-bit API

```
    movl %eax, (%esi, %ecx, 8)
    movl %ebx, 4(%esi, %ecx, 8)
```

vs

```
    movl %rax, (%rsi, %rcx, 8)
    movl %rbx, 8(%rsi, %rcx, 8)
```

https://review.coreboot.org/c/coreboot/+/81968/comment/78354408_691cbdb3?usp... :
PS64, Line 87: 	jmp leave
> I don't think you can just leave here? You're now pointing the PML4E straight at the PDE, but you st […]

Acknowledged