Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add kernel functions for 128-bit long doubles. These could be improved
a bit, but access to a freebsd/sparc64 machine is needed.

Submitted by: bde and Steve Kargl <sgk@apl.washington.edu> (earlier version)