Fix x86_64 rawmemchr namespace (bug 17572).

Fix x86_64 rawmemchr namespace (bug 17572).

rawmemchr is not an ISO C function, but __rawmemchr is called from ISO
C functions, so rawmemchr should be a weak alias.  On most
architecture it is, but x86_64 defines the function as rawmemchr with
__rawmemchr as a strong alias.  This patch makes x86_64 follow the
same arrangements as other architectures.

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

	[BZ #17572]
	* sysdeps/x86_64/rawmemchr.S (rawmemchr): Rename to __rawmemchr
	and define as weak alias of __rawmemchr.
	(__rawmemchr): Do not define as strong alias of rawmemchr.