Disable amd64 TLB Context ID (pcid) by default for now

There are a number of reports of userspace application crashes that
are "solved" by setting vm.pmap.pcid_enabled=0, including Java and the
x11/mate-terminal port (PR ports/184362).

For now apply a band-aid of disabling pcid by default in stable/10.

Sponsored by:	The FreeBSD Foundation