Add memcpy/memmove/memset benchmarks with large data

Add memcpy, memmove and memset benchmarks with large data sizes.

	* benchtests/Makefile (string-benchset): Add memcpy-large,
	memmove-large and memset-large.
	* benchtests/bench-memcpy-large.c: New file.
	* benchtests/bench-memmove-large.c: Likewise.
	* benchtests/bench-memmove-large.c: Likewise.
	* benchtests/bench-string.h (TIMEOUT): Don't redefine.