Avoid race conditions when rebuilding librt.so

`make check' sometimes triggers a rebuild of librt.so using
nptl/Makefile, which ignores librt's dependence on libpthread.  This
causes the build to blow up when we attempt to run the test suite on
RISC-V.

2018-01-06  Palmer Dabbelt  <palmer@sifive.com>

        * nptl/Makefile (/librt.so): Always depend on
        "$(shared-thread-library)".