Add AVX and AVX2/BMI implementations for SHA-256
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Add AVX and AVX2/BMI implementations for SHA-256

* LICENSES: Add 'cipher/sha256-avx-amd64.S' and
'cipher/sha256-avx2-bmi2-amd64.S'.
* cipher/Makefile.am: Add 'sha256-avx-amd64.S' and
'sha256-avx2-bmi2-amd64.S'.
* cipher/sha256-avx-amd64.S: New.
* cipher/sha256-avx2-bmi2-amd64.S: New.
* cipher/sha256-ssse3-amd64.S: Use 'lea' instead of 'add' in few
places for tiny speed improvement.
* cipher/sha256.c (USE_AVX, USE_AVX2): New.
(SHA256_CONTEXT) [USE_AVX, USE_AVX2]: Add 'use_avx' and 'use_avx2'.
(sha256_init, sha224_init) [USE_AVX, USE_AVX2]: Initialize above
new context members.
[USE_AVX] (_gcry_sha256_transform_amd64_avx): New.
[USE_AVX2] (_gcry_sha256_transform_amd64_avx2): New.
(transform) [USE_AVX2]: Use AVX2 assembly if enabled.
(transform) [USE_AVX]: Use AVX assembly if enabled.
* configure.ac: Add 'sha256-avx-amd64.lo' and
'sha256-avx2-bmi2-amd64.lo'.

Patch adds fast AVX and AVX2/BMI2 implementations of SHA-256 by Intel
Corporation. The assembly source is licensed under 3-clause BSD license,
thus compatible with LGPL2.1+. Original source can be accessed at:
http://www.intel.com/p/en_US/embedded/hwsw/technology/packet-processing#docs

Implementation is described in white paper
"Fast SHA - 256 Implementations on Intel® Architecture Processors"
http://www.intel.com/content/www/us/en/intelligent-systems/intel-technology/sha-256-implementations-paper.html

Note: AVX implementation uses SHLD instruction to emulate RORQ, since it's

faster on Intel Sandy-Bridge. However, on non-Intel CPUs SHLD is much
slower than RORQ, so therefore AVX implementation is (for now) limited
to Intel CPUs.

Note: AVX2 implementation also uses BMI2 instruction rorx, thus additional