gallium/util: Fix detection of AVX cpu caps

gallium/util: Fix detection of AVX cpu caps

For AVX it's not sufficient to only rely on the cpuid flags. If the CPU
supports these extensions, but the OS doesn't, issuing these insns will
trigger an undefined opcode exception.

In addition to the AVX cpuid bit we also need to:
* test cpuid for OSXSAVE support
* XGETBV to check if the OS saves/restores AVX regs on context switches

See "Detecting Availability and Support" at
http://software.intel.com/en-us/articles/introduction-to-intel-advanced-vector-extensions

Signed-off-by: Andre Heider <a.heider@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>