* 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.

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)