cpu/x86/mp: pass pointers to structures for AP callbacks

cpu/x86/mp: pass pointers to structures for AP callbacks

In order to extend the MP callback infrastructure prepare for
easier changes by making the AP callback get signalled by a
single pointer to a local variable on the signaller's stack.
When the APs see the callback they will copy the structure
to a local variable and then set the acknowledgement by
clearing out the slot.

The reading and writing to the slots were implemented using inline
assembly which forces a memory access and a compiler barrier.

BUG=b:74436746

Change-Id: Ia46133a49c03ce3ce0e73ae3d30547316c7ec43c
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/26043
Reviewed-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>