head/sys/amd64/amd64/support.S: unroll loop
unroll the loop in ENTRY(pagezero)
acc' to the submitter this results in a reproducible 1% perf
improvement under buildworld like workload

I validated correctness and run-testing, but not performance impact

Submitted by: lidl@pix.net
Reviewed by: adrian
PR: 199151
MFC After: 1 month