[Bug libc/20689] Test for FMA should also check for AVX.

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https://sourceware.org/bugzilla/show_bug.cgi?id=20689

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=b3d17c1cf29ac8cd6b2fcd1db7d824e4af9ea505

commit b3d17c1cf29ac8cd6b2fcd1db7d824e4af9ea505
Author: Carlos O'Donell <carlos@systemhalted.org>
Date:   Mon Oct 17 19:35:34 2016 -0400

    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

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                  |    6 ++++++
 sysdeps/x86/cpu-features.c |   24 ++++++++++++++----------
 2 files changed, 20 insertions(+), 10 deletions(-)

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