Fix gethostbyaddr in6addr_any, in6addr_loopback namespace (bug 18532).

Fix gethostbyaddr in6addr_any, in6addr_loopback namespace (bug 18532).

gethostbyaddr brings in references to in6addr_any and thereby
in6addr_loopback, which aren't in all the standards containing
gethostbyaddr (gethostbyaddr is in XPG4 and UNIX98, in6addr_any and
in6addr_loopback are new in POSIX.1:2001).  This patch fixes this by
making those symbols into weak aliases (safe in this case, unlike for
most data symbols, because these data symbols are const).

Tested for x86_64 and x86 (testsuite, and comparison of disassembly of
installed stripped shared libraries).  Disassembly is unchanged for
x86_64; for x86, I see some changes of stack offsets, but no other
code generation changes or code size differences.

	[BZ #18532]
	* inet/in6_addr.c (in6addr_any): Rename to __in6addr_any and
	define as weak alias of __in6addr_any.  Use libc_hidden_data_weak.
	(in6addr_loopback): Rename to __in6addr_loopback and define as
	weak alias of __in6addr_loopback.  Use libc_hidden_data_weak.
	* include/netinet/in.h (__in6addr_loopback): Declare.  Use
	libc_hidden_proto.
	(__in6addr_any): Likewise.
	* inet/gethstbyad_r.c (PREPROCESS): Use __in6addr_any instead of
	in6addr_any.
	* conform/Makefile (test-xfail-XPG4/netdb.h/linknamespace): Remove
	variable.
	(test-xfail-UNIX98/netdb.h/linknamespace): Likewise.