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

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

Remove an unused variable.

Sigh, the weak reference for ceill(), floorl() and truncl() was in
unreachable code due to a missing include. This kept arm and powerpc
broken.

Reported by: sam, grehan

Oops, the weak reference for ceill(), floorl() and truncl() was in the
wrong file. This broke arm and powerpc.

Reported by: grehan

Implement trunc() and truncf().