Fix sem_* tdelete, tfind, tsearch, twalk namespace (bug 18536).

Fix sem_* tdelete, tfind, tsearch, twalk namespace (bug 18536).

The sem_* functions bring in references to tdelete, tfind, tsearch and
twalk.  But the t* functions are XSI-shaded, while sem_* aren't.  This
patch fixes this by using __t* instead, exporting those functions from
libc at version GLIBC_PRIVATE (since sem_* are in libpthread) and
using libc_hidden_* for the benefit of calls within libc.

Tested for x86_64 and x86 (testsuite, and comparison of disassembly of
installed stripped shared libraries).  libpthread gets changes from
PLT reordering; addresses in libc change because of PLT / dynamic
symbol table changes.

	[BZ #18536]
	* misc/tsearch.c (__tsearch): Use libc_hidden_def.
	(__tfind): Likewise.
	(__tdelete): Likewise.
	(__twalk): Likewise.
	* misc/Versions (libc): Add __tdelete, __tfind, __tsearch and
	__twalk to GLIBC_PRIVATE.
	* include/search.h (__tsearch): Use libc_hidden_proto.
	(__tfind): Likewise.
	(__tdelete): Likewise.
	(__twalk): Likewise.
	* nptl/sem_close.c (sem_close): Call __twalk instead of twalk.
	Call __tdelete instead of tdelete.
	* nptl/sem_open.c (check_add_mapping): Call __tfind instead of
	tfind.  Call __tsearch instead of tsearch.
	* sysdeps/sparc/sparc32/sem_open.c (check_add_mapping): Likewise.
	* conform/Makefile (test-xfail-POSIX/semaphore.h/linknamespace):
	Remove variable.
	(test-xfail-POSIX2008/semaphore.h/linknamespace): Likewise.