Import xz-embedded from git.

This is from commit hash '6a8a2364434763a033781f6b2a605ace9a021013'.
MFV r262756:

Import xz-embedded from git.

This is from commit hash '6a8a2364434763a033781f6b2a605ace9a021013'.

This makes it possible to use CRC64 but for now it's intentionally
not added to build.
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).

I originally planned to disable this only in stable/10 (in r262753), but
it has been pointed out that additional crash reports on HEAD are not
likely to provide new insight into the problem.  The feature can easily
be enabled for testing.