binutils/ld: Don't add /usr/lib to the library search path twice.
The various toolchains we build during buildworld need different
search paths for various stages. The ${USRDATA_PREFIX} variable,
which has different values depending on the stage, is used as a
prefix so that the compilers we use to build our world can search
their includes and libraries in /usr/obj. For the compiler that
ultimately gets installed upon installworld, the prefix is empty.
In ld's Makefile, the correct path prefixed with ${USRDATA_PREFIX}
is already added to the default path a few lines above, so all is
good. This commit removes the extra /usr/lib that was unconditionally
added after the prefixed one (which is wrong).