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

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

Back out r255440. /usr/bin/gcc @r255185 (2013-09-03) can build this.

Approved by: re (kib)

Only use a clang'ism if ${CC} is clang.

Reviewed by: sjg
Approved by: re (kib)

Use the fact that the AES-NI instructions can be pipelined to improve
performance... Use SSE2 instructions for calculating the XTS tweek
factor... Let the compiler do more work and handle register allocation
by using intrinsics, now only the key schedule is in assembly...

Replace .byte hard coded instructions w/ the proper instructions now
that both clang and gcc support them...

On my machine, pulling the code to userland I saw performance go from
~150MB/sec to 2GB/sec in XTS mode. GELI on GNOP saw a more modest
increase of about 3x due to other system overhead (geom and
opencrypto)...

These changes allow almost full disk io rate w/ geli...

Reviewed by: -current, -security
Thanks to: Mike Hamburg for the XTS tweek algorithm

style.Makefile(5)

Crypto(4) driver for AESNI.

The aeskeys_{amd64,i386}.S content was mostly obtained from OpenBSD,
no objections to the license from core.

Hardware provided by: Sentex Communications
Tested by: fabient, pho (previous versions)
MFC after: 1 month