Fix C11 conformance issues

Remove conformace assumption of NPTL implementation for ISO C threads
and revert wrong libcrypt addition on linknamespace-libs-XPG4.

The i686-gnu target now shows two new conformance failures:

FAIL: conform/ISO11/threads.h/conform
FAIL: conform/ISO11/threads.h/linknamespace

It is expected due missing HTL ISO C threads support and both conformance
.out files indicates the reason ("#error "HTL does not implement ISO C
threads").

Checked on i686-linux-gnu and i686-gnu.

	* include/threads.h: Move to ...
	* sysdeps/nptl/threads.h: ... here.
	* sysdeps/htl/threads.h: New file.
	* conform/Makefile (linknamespace-libs-ISO11): Use
	static-thread-library instead of linking libpthread.
	(linknamespace-libs-XPG4): Revert wrong libcrypt.a addition.