Change in coreboot[master]: *x86: Support x2apic mode

[Nico Huber (Code Review)]

Attention is currently required from: Furquan Shaikh, Jamie Ryu, Wonkyu Kim, Paul Menzel, Angel Pons, Subrata Banik, Raj Astekar, Patrick Rudolph.
Nico Huber has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/51723 )

Change subject: *x86: Support x2apic mode
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Patch Set 6:

(2 comments)

File src/cpu/x86/lapic/lapic_cpu_init.c:

PS6: 
I'm not 100%, but I thought the code here was abandoned in favor
of the mp_init version. If this is the case, the changes here are
probably untested and belong into a separate commit.

File src/cpu/x86/mp_init.c:

https://review.coreboot.org/c/coreboot/+/51723/comment/dc06e117_cb53ff62 
PS6, Line 458: 	} else {
> Return 0 in the if-block and drop the `else`? This way, you don't need to reindent everything.

Or even better, add more functions. e.g. start_aps_lapic(), start_aps_lx2apic().
Only the call would have to be inside an if.

With a forward declaration the original code could even stay in
place, i.e. wouldn't show up much in the diff. If you want to
move it, that could be done in a later commit.