libthr: move _umtx_op_err() to libsys

Declare in sys/umtx.h and implement in libsys. Explicitly link libthr
with libsys.

When building libthr static include _umtx_op_err so we don't break static
linkage with -lpthread.

Reviewed by: kib, emaste, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/908