[build] Remove obsolete check for GNU as version 2.9.1

Reported-by: Dewey Hylton <dewey@hyltown.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[ipv6] Fix compilation under FreeBSD

Reported-by: Dewey Hylton <dewey@hyltown.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>