Bug 20689: Fix FMA and AVX2 detection on Intel

In the Intel Architecture Instruction Set Extensions Programming
reference the recommended way to test for FMA in section
'2.2.1 Detection of FMA' is:

"Application Software must identify that hardware supports AVX as
explained in ... after that it must also detect support for FMA..."

We don't do that in glibc. We use osxsave to detect the use of xgetbv,
and after that we check for AVX and FMA orthogonally. It is conceivable
that you could have the AVX bit clear and the FMA bit in an undefined
state.

This commit fixes FMA and AVX2 detection to depend on usable AVX
as required by the recommended Intel sequences.

v1: https://www.sourceware.org/ml/libc-alpha/2016-10/msg00241.html
v2: https://www.sourceware.org/ml/libc-alpha/2016-10/msg00265.html