Declare memccpy, strdup, strndup for C2X.

C2X adds the memccpy, strdup and strndup functions.  This patch duly
adds __GLIBC_USE (ISOC2X) to the conditions under which <string.h>
declares them.

Tested for x86_64.

	* string/string.h (memccpy): Also declare if [__GLIBC_USE (ISOC2X)].
	(strdup): Likewise.
	(strndup): Likewise.