This avoids a segfault like the following for at least some 4.8 versions
of gcc when configured with --static if avx2 instructions are also
enabled:

	Program received signal SIGSEGV, Segmentation fault.
	buffer_find_nonzero_offset_ifunc () at ./util/cutils.c:333
	333     {
	(gdb) bt
	#0  buffer_find_nonzero_offset_ifunc () at ./util/cutils.c:333
	#1  0x0000000000939c58 in __libc_start_main ()
	#2  0x0000000000419337 in _start ()

Signed-off-by: Aaron Lindsay <alindsay@codeaurora.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>