reading cr.ivr, as well as writing to cr.eoi.
o use global variables to pass information to os_boot_rendez
so that it doesn't have to jump through hoops to find it
out. This avoids traps on the AP without it even being
initialized. This fixes SMP configurations.
o Move the probing of the MADT to the end of cpu_startup,
instead of at the start of cpu_mp_probe. We need to probe
the MADT for non-SMP configurations as well. This fixes
uniprocessor configurations.
o Serialize AP wake-up by waiting for the AP. We need to do
this since we use global variables to for the AP to use.
As a side-effect, we can use printf() more easily to see
what's going on.
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