Some architectures, in this case powerpc64, need explicit synchronization
barriers vs device accesses.
Prior to this change, when running 'make buildworld -j72' on a 18-core
(72-thread) POWER9, I would see "Missing interrupt" errors and controller resets
often. Also, due to the missing interrupts and resets, it would take ~13
minutes to build world. After this change the "Missing interrupt" messages do
not appear, and buildworld times are 10-11 minutes.
The placement of the syncs may not be 100% correct.