configure: Make AVX2 test robust to non-ELF systems

The AVX2 optimization test assumes that the object format
is ELF and the system has the readelf utility. If this isn't
true then configure might fail or emit a warning (since in
a pipe "foo | bar >/dev/null 2>&1" does not redirect the
stderr of foo, only of bar). Adjust the check so that if
we don't have readelf or don't have an ELF object then we
just don't enable the AVX2 optimization.

Reported-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Message-id: 1466287502-18730-3-git-send-email-pmaydell@chiark.greenend.org.uk
