libthr: filter rather than link with libsys

The allows gcc + GNU ld to link programs with -m32 -pthread without
erroring out due to _umtx_op_err being undefined (unless -lsys is added
to the link command.

We now always link _umtx_op_err into libthr (not just when it's static)
and filter it with libsys so we call that implementation. The dynamic
implementations (at least the assembly ones) should likely become stubs
as a further refinement.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D43783