Add reachover Makefiles for contrib/netbsd-tests/lib/libpthread as
lib/libthr/tests

A variant of this code has been tested on amd64/i386 for some time by
EMC/Isilon on 10-STABLE/11-CURRENT. It builds on other architectures, but the
code will remain off until it's proven it works on virtual hardware or real
hardware on other architectures

Original work by: pho
Sponsored by: EMC / Isilon Storage Division