thread libs: move libc_r and libthread_xu to /usr/lib/thread

thread libs: move libc_r and libthread_xu to /usr/lib/thread

This is so that badly written programs don't mistakenly pick up either
libc_r or libthread_xu.  The only correct way to get threading support
is to link to libpthread.

By moving the underlaying thread libs to a directory that is not
searched by the linker, we can enforce this policy.

Requested-by:  hasso@