Fix __printf_fp wmemset namespace (bug 17574).

Fix __printf_fp wmemset namespace (bug 17574).

__printf_fp calls wmemset, but that is not an ISO C90 function.  This
patch fixes it to call a new __wmemset name instead (with wmemset
being a weak alias).

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

	[BZ #17574]
	* wcsmbs/wmemset.c (wmemset): Rename to __wmemset and define as
	weak alias of __wmemset.  Use libc_hidden_weak.
	(__wmemset): Use libc_hidden_def.
	* include/wchar.h (__wmemset): Declare.  Use libc_hidden_proto.
	* stdio-common/printf_fp.c (___printf_fp): Call __wmemset instead
	of wmemset.