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

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

Add section .note.GNU-stack for assembly files used by 386 and amd64.

Use, in uncovered part, the END() macro in order to improve debugging.
In this specific case, Valgrind won't get confused when analyzing such
functions.

Sponsored by: Sandvine Incorporated
Tested by: emaste
MFC: 3 days

Import the subset of J.T. Conklin's single-precision x86-optimized
math routines that appear to be (a) correct and (b) faster than their
MI counterparts on my Pentium 4.

Obtained from: NetBSD