Improve string benchtests

Replace slow byte-oriented tests in several string benchmarks with the
generic implementations from the string/ directory so the comparisons
are more realistic and useful.

	* benchtests/bench-stpcpy.c (SIMPLE_STPCPY): Remove function.
	(generic_stpcpy): New function.
	* benchtests/bench-stpncpy.c (SIMPLE_STPNCPY): Remove function.
	(generic_stpncpy): New function.
	* benchtests/bench-strcat.c (SIMPLE_STRCAT): Remove function.
	(generic_strcat): New function.
	* benchtests/bench-strcpy.c (SIMPLE_STRCPY): Remove function.
	(generic_strcpy): New function.
	* benchtests/bench-strncat.c (SIMPLE_STRNCAT): Remove function.
	(STUPID_STRNCAT): Remove function.
	(generic_strncat): New function.
	* benchtests/bench-strncpy.c (SIMPLE_STRNCPY): Remove function.
	(STUPID_STRNCPY): Remove function.
	(generic_strncpy): New function.
	* benchtests/bench-strnlen.c (SIMPLE_STRNLEN): Remove function.
	(generic_strnlen): New function.
	(memchr_strnlen): New function.
	* benchtests/bench-strlen.c (generic_strlen): Define for WIDE.
	(memchr_strlen): Likewise.