Various changes to pmap_ts_referenced()

Move PMAP_TS_REFERENCED_MAX out of the various pmap implementations and
into vm/pmap.h, and describe what its purpose is.  Eliminate the archaic
"XXX" comment about its value.  I don't believe that its exact value, e.g.,
5 versus 6, matters.

Update the arm64 and riscv pmap implementations of pmap_ts_referenced()
to opportunistically update the page's dirty field.

On amd64, use the PDE value already cached in a local variable rather than
dereferencing a pointer again and again.