htl: Enable more cancel tests

* nptl/tst-cancel11.c, tst-cancel21-static.c, tst-cancel21.c, tst-cancel6.c, tst-cancelx11.c, tst-cancelx21.c, tst-cancelx6.c: Move to...
* sysdeps/pthread: ... here.
* nptl/Makefile: Move corresponding references and rules to...
* sysdeps/pthread/Makefile: ... here.