Move semaphore.h to sysdeps/pthread/.

Move semaphore.h to sysdeps/pthread/.

Carlos reported failures in conform/ tests in environments where the
compiler used could only find headers in glibc's source and build
trees, not any previously installed headers
<https://sourceware.org/ml/libc-alpha/2014-09/msg00040.html>.

This patch moves nptl/semaphore.h to sysdeps/pthread/semaphore.h so
that it can be found by builds from all glibc subdirectories; it's not
in any way NPTL-specific.  (I left the Makefile setting to install
this header in nptl/, but maybe it should move as well - it's just not
clear to me what ifeq ($(subdir),...) conditional should be used to
select the directory to associate the header with for installation
purposes.  The path in the toplevel Makefile used for begin-end-check
also remains hardcoded; it's a known todo issue to rework that test to
run in each subdirectory checking the headers installed from that
subdirectory, rather than a separate hardcoded list.)

Tested for x86_64 (testsuite, and that installed stripped shared
libraries are unchanged by the patch).  I did *not* test a
configuration such as that in which Carlos saw failure.

	* nptl/semaphore.h: Move to ...
	* sysdeps/pthread/semaphore.h: ... here.
	* Makefile (installed-headers): Change nptl/semaphore.h to
	sysdeps/pthread/semaphore.h.