fix crash on CPU not having AVX support

Hi,
the _xgetbv() instruction lead to a crash on my laptop (i7 Q740), producing a "illegal instruction" error.
It seems some checks have to be performed before calling this instruction, I have added them and included them below.

Regards,
Gregory

These bools testing bits 27 and 28 are a bit redundant, it would be cleaner to use the already defined AVXFlag mask with a test like "if ( (cpuInfo[2] & AVXFlag) == AVXFlag )".  Good fix though, I have the same crash.