NetBSD/MIPS supports dlopen

Hi,

Recent version of NetBSD/MIPS support dlopen. This patch makes the
netbsd dynloader tests availability of dlopen on HAVE_DLOPEN rather
than on __mips__

Tested on NetBSD 4.99.20 on mips

I plan on registering a buildfarm member once this patch is in (and
maybe after upgrading to a more current NetBSD-current).

Regards,

Rémi Zara