apparently, this causes some quadcore ramnode vm to hang on boot,
even tho all cores successfully started up and are operational.
i suspect some side effect from timersinit()... this would also
mean *notsc= would break it (syncclock() would continue)...
its unclear.
i'm reverting this for now until the problem is better understood.
when testing in qemu, launching each ap became slower and slower
because all the ap's where spinning in syncclock() waiting for
cpu0 to update its mach0->tscticks, which happens only much later
after all cpu's have been started up.
now we wait for each cpu to do its timer callibration and
manually update our tscticks while we wait and each cpu will
not spin but halt while waiting for active.thunderbirdsarego.
this reduces the system load and noise for timer callibration
and makes the mp startup linear with regard to the number of
cores.
to make it possible to mark the bootscreen framebuffer
as write combining in early initialization, mtrr() is
changed not not to error() but to return an error string.
as bootscreen() is used before multiprocessor initialization,
we have to synchronize the mtrr's for every processor as
it comes online. for this, a new mtrrsync() function is
provided that is called from cpuidentify() if mtrr support
is indicated.
the boot processor runs mtrrsync() which snarfs the
registers. later, mtrrsync() is run again from the
application processors which apply the values from the
boot processor.
checkmtrr() from mp.c was removed as its task is also
done by mtrrsync() now.
