Load libc.so.7 instead of .6 on FreeBSD

Loading libc.so.6 results in a bazillion warnings like this:

> ruby19 in free(): warning: junk pointer, too high to make sense

Perhaps memory malloc()ed by libc.so.7 is attempted to be free()d via the libc.so.6 loaded afterwards.  In any case, everything seems to work fine with libc.so.7.