Fix ISO C threads installed header and HURD assumption

Set installed NPTL header as the expected one (instead of an
internal one for glibc testsuite) and add a hurd specific
stdc-predef with __STDC_NO_THREADS__.

Checked on both i686-linux-gnu and i686-gnu that both threads.h
and stdc-predef.h are the expected ones.

	* nptl/threads.h: Move to ...
	* sysdeps/nptl/threads.h: ... here.
	* sysdeps/hurd/stdc-predef.h: New file.