pthread: Move back linking rules to nptl and htl

d6d74ec1 ('htl: Enable more tests') moved the linking rules from
nptl/Makefile and htl/Makefile to the shared sysdeps/pthread/Makefile.  But
e.g. on powerpc some tests are added in sysdeps/powerpc/Makefile, which is
included *after* sysdeps/pthread/Makefile, and thus the tests don't get
affected by the rules and fail to link.  For now let's just copy over the
set of rules in both nptl/Makefile and htl/Makefile.

* sysdeps/pthread/Makefile: Move libpthread linking rules to...
* htl/Makefile: ... here and...
* nptl/Makefile: ... there.