Disable bsd-user binaries on NetBSD 5.x and prior for workaround.

i386 and x86_64 user emul binaries require newer binutils for PROVIDE_HIDDEN()
http://git.qemu.org/qemu.git/commit/?id=845f2c2812d9ed24b36c02a3d06ee83aeafe8b49
but on NetBSD binutils-2.19 was enabled around 5.99.22:
http://cvsweb.NetBSD.org/bsdweb.cgi/src/share/mk/bsd.own.mk#rev1.594
and qemu-0.13.0 build fails on NetBSD 5.1 and 5.0.2.

Note qemu, qemu-system-x86_64, and other non user binaries
(that would actually be required by most users) still work fine.

(tsutsui)