Fix x86_64 memrchr namespace (bug 17719).

On x86_64, memrchr (not a standard function) is defined as a strong
symbol, instead of a weak alias of __memrchr as on other
architectures.  This results in linknamespace test failures from the
use of __memrchr from dirname.  (Not a conformance issue because of
the mem* reservation, but contrary to glibc conventions.)  This patch
makes x86_64 follow other architectures by defining memrchr as a weak
alias.

Tested for x86_64 (testsuite, and that disassembly of installed shared
libraries is unchanged by the patch).

	[BZ #17719]
	* sysdeps/x86_64/memrchr.S (memrchr): Rename to __memrchr and
	define as weak alias of __memrchr.
	(__memrchr): Do not define as strong alias of memrchr.
	* conform/Makefile (test-xfail-XPG4/libgen.h/linknamespace):
	Remove variable.
	(test-xfail-UNIX98/libgen.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K/libgen.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K8/libgen.h/linknamespace): Likewise.