Parse /proc/cpuinfo for ARM HW features

* src/hwf-arm.c [__linux__] (HAS_PROC_CPUINFO)
(detect_arm_proc_cpuinfo): New.
(_gcry_hwf_detect_arm) [HAS_PROC_CPUINFO]: Check '/proc/cpuinfo' for
HW features.
--

Some Linux platforms (read: Android) block read access to '/proc/self/auxv',
which prevents NEON HW detection. Patch adds alternative check which parses
'/proc/cpuinfo' which should be accessable by Android applications.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Add AMD64 assembly implementation for arcfour

* cipher/Makefile.am: Add 'arcfour-amd64.S'.
* cipher/arcfour-amd64.S: New.
* cipher/arcfour.c (USE_AMD64_ASM): New.
[USE_AMD64_ASM] (ARCFOUR_context, _gcry_arcfour_amd64)
(encrypt_stream): New.
* configure.ac [host=x86_64]: Add 'arcfour-amd64.lo'.
--

Patch adds Marc Bevand's public-domain AMD64 assembly implementation of RC4 to
libgcrypt. Original implementation is at:
  http://www.zorinaq.com/papers/rc4-amd64.html

Benchmarks on Intel i5-4570 (3200 Mhz):

New:
 ARCFOUR        |  nanosecs/byte   mebibytes/sec   cycles/byte
     STREAM enc |      1.29 ns/B     737.7 MiB/s      4.14 c/B
     STREAM dec |      1.31 ns/B     730.6 MiB/s      4.18 c/B

Old (C-language):
 ARCFOUR        |  nanosecs/byte   mebibytes/sec   cycles/byte
     STREAM enc |      2.09 ns/B     457.4 MiB/s      6.67 c/B
     STREAM dec |      2.09 ns/B     457.2 MiB/s      6.68 c/B

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>