bootstrap/6877: need to manually make libiberty

[DJ Delorie]

> Interestingly, that seems to work (even after cleaning things out).  Odd
> that "make install" isn't just equivalent to "make all ; make install"

If you have a unified source tree, "make all" takes a long time, even
if it's all up to date.  Besides, "install" is a command, not a
target, like "clean" or "distrib".  You wouldn't want "make clean" to
do a "make all" first, would you?  (not that that's a good example, of
course).