just for whom it may concern: I've rolled back r326619 and reapplied slw patch on top - again, on "works for me" basics.

"works for me"
(survived make buildworld two times in a row ;-) kernel from clean sources hangs, as usual, somewhere in stage3/clang)

somehow this patch solved my old problem - instant hangs somewhere in building libclang/libllvm while making buildworld.
my configuration: 11stable x64 on vmware vm , zfs (default layout by bsdinstall) 4 or 6G memory (no visible difference, no explicit prefetch on 4G setup), 4core parallel build
top usually reports something like "2G free" (until entire vm become nonresponsible)