Makefile: Don't insist on `/usr/lib/.../coreutils/'.

Makefile: Don't insist on `/usr/lib/.../coreutils/'.

It turns out that sid has moved `stdlib.so' to `/usr/libexec/', which
sucks for two reasons: (a) it means we need countermeasures to avoid
the cross-tools extraction from failing, and (b) there's no multiarch
triplet any more so we can't have multiple versions.

I think this is actually a Debian bug, but I have to deal with it right
now and this seems like the least terrible approach.  I'll just hope
that nothing is relying on `stdbuf' in the build scripts.