[S] Change in coreboot[master]: cpu/x86/smm_stub.S: Add a retpoline around the main C handler

[Maximilian Brune (Code Review)]

Attention is currently required from: Nico Huber, Martin L Roth, Subrata Banik, Patrick Rudolph, Benjamin Doron, Julius Werner, Arthur Heymans.

Maximilian Brune has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/67735 )

Change subject: cpu/x86/smm_stub.S: Add a retpoline around the main C handler
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Patch Set 2:

(1 comment)

File src/cpu/x86/smm/smm_stub.S:

https://review.coreboot.org/c/coreboot/+/67735/comment/933a08ec_f01a5bb6 
PS2, Line 256: jmp	2f
would it make sense to do:
call call_c_handler
here and leave the 1: and 2: label code out? So it would basically result in this:
```
    call	call_c_handler
trap:
    pause
    lfence
    jmp	trap
call_c_handler:
    movq	%rax, (%rsp)
    ret
    int3
```