Refactor string benchtests

Refactor string benchtests

Refactor string benchtests by moving duplicated defines into
bench-string.h.

	* benchtests/bench-memchr.c: Cleanup defines.
	* benchtests/bench-memcmp.c: Likewise.
	* benchtests/bench-memset.c: Likewise.
	* benchtests/bench-memset-large.c: Likewise.
	* benchtests/bench-memset-walk.c: Likewise.
	* benchtests/bench-stpcpy.c: Likewise.
	* benchtests/bench-stpncpy.c: Likewise.
	* benchtests/bench-strcat.c: Likewise.
	* benchtests/bench-strchr.c: Likewise.
	* benchtests/bench-strcmp.c: Likewise.
	* benchtests/bench-strcpy.c: Likewise.
	* benchtests/bench-strcspn.c: Likewise.
	* benchtests/bench-string.h: Likewise.
	* benchtests/bench-strlen.c: Likewise.
	* benchtests/bench-strncat.c: Likewise.
	* benchtests/bench-strncmp.c: Likewise.
	* benchtests/bench-strncpy.c: Likewise.
	* benchtests/bench-strnlen.c: Likewise.
	* benchtests/bench-strpbrk.c: Likewise.
	* benchtests/bench-strrchr.c: Likewise.
	* benchtests/bench-strspn.c: Likewise.