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

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

Merge in the latest SoftFloat changes from NetBSD. (NetBSD isn't the
original vendor, but we're using their heavily modified version.)
This brings in functions for long double emulation (both extended and
quad formats), which may be useful for testing, and also for replacing
libc/sparc64/fpu/.