Fix gcc build errors with SSE 4.2 detection and gcc warnings

Fix gcc build errors with SSE 4.2 detection and gcc warnings

- Remove -Wunused-but-set-variable's (`len`, etc).
- Replace clang-specific tests in sse42_supported(..) with generic,
  FreeBSD-supported CPU feature tests, using macros and functions
  from machine/cpufunc.h and machine/specialreg.h . The previous method
  for determining SSE4.2 availability was only compatible with clang.
- Sort #includes per style(9).

Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D7119
MFC after: 1 week
X-MFC with: r302332
Reported by: Jenkins
Reviewed by: grehan (maintainer)
Sponsored by: EMC / Isilon Storage Division