Make avx2 configure test work with -O2

When configured with --extra-cflags=-O2 gcc optimised out the test
and the readelf failed the check leaving avx2 disabled.
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <1465557378-24105-2-git-send-email-dgilbert@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -1781,7 +1781,7 @@ fi
 cat > $TMPC << EOF
 static void bar(void) {}
 static void *bar_ifunc(void) {return (void*) bar;}
-static void foo(void) __attribute__((ifunc("bar_ifunc")));
+void foo(void) __attribute__((ifunc("bar_ifunc")));
 int main(void) { foo(); return 0; }
 EOF
 if compile_prog "-mavx2" "" ; then