htl: Enable cancel*16 an cancel*20 tests

* nptl/tst-cancel16.c, tst-cancel20.c, tst-cancelx16.c, tst-cancelx20.c:
Move to...
* sysdeps/pthread: ... here.
* nptl/Makefile: Move corresponding references and rules to...
* sysdeps/pthread/Makefile: ... here.
* sysdeps/mach/hurd/i386/Makefile: Xfail tst-cancel*16 for now: missing
barrier pshared support, but test should be working otherwise.