cpplib: basename () fix

[DJ Delorie]

> In the basename case, we don't need a fallback because we just use the
> libiberty version when cross compiling (ie., test fails) unless the
> runtime environment explicitly excludes it from the list of funcs to
> check.

And I say again, we might as well just always build and include our
own basename then.  Alloca is different in that, even when we build
our own, it must know something about the runtime environment just to
function properly.  However, it is equipped to defer that test until
runtime.  We can't defer linking basename or not until after runtime.