stablize UP USIII support by blocking interrupts around sp_tlb_flush_pte()
i was seeing stack corruption while taking an interrupt in this function.

get USIII SMP mostly working by implementing the cheetah version of
sparc64_ipi_flush_pte().

SMP support is still not entirely stable.  i can reproducably get a:

panic: fpusave_lwp ipi didn't

while running build.sh, when an awk process is exiting.  other simple
heavy work loads do not crash for me right now.

(mrg)