Fix buggy/incomplete detection of AVX/AVX2 support

* configure.ac: Also check for 'xgetbv' instruction in AVX and AVX2
inline assembly checks.
* src/hwf-x86.c [__i386__] (get_xgetbv): New function.
[__x86_64__] (get_xgetbv): New function.
[HAS_X86_CPUID] (detect_x86_gnuc): Check for OSXSAVE and OS support for
XMM&YMM registers and enable AVX/AVX2 only if XMM&YMM registers are
supported by OS.
--

This patch is based on original patch and bug report by Panagiotis Christopoulos:

  Adding better detection of AVX/AVX2 support

  After upgrading libgcrypt from 1.5.3 to 1.6.0 on a remote XEN system (linode) my
  gpg2 stopped working properly, throwing SIGILL signals when doing sha512
  operations etc. I managed to debug this with the help of Doublas Freed
  (dwfreed at mtu.edu) and it seems that the current AVX detection just checks for
  bit 28 on cpuid but the check still works  on systems that have disabled the avx/avx2
  instructions for some reason (eg. performance/unstability) resulting in SIGILLs
  (eg. when trying _gcry_sha512_transform_amd64_avx() ).
  From Intel resources[1][2], I found additional checks for better AVX
  detection and applied them in the following patch. Please review/change
  accordingly and commit some better AVX detection mechanism. The AVX part is
  tested but could not test the AVX2 one, because I lack proper hardware. I can
  provide additional information upon request. Use the patch only as a guideline,
  as it's not thoroughly tested.

  [1] http://software.intel.com/en-us/blogs/2011/04/14/is-avx-enabled
  [2] http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-manual-325462.pdf (sections 14.3 and 14.7.1)

Reported-by: Panagiotis Christopoulos (pchrist) <pchrist@gentoo.org>
Cc: Doublas Freed <dwfreed@mtu.edu>
Cc: Tim Harder <radhermit@gentoo.org>
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>