Implement write alignment of 64, add macro parameters, change benchmarks

Implement write alignment of 64 in the memcpy_simple variants.

The CHECK_SMALL_SIZE_ALIGNMENT and PRELOAD_CATCH_UP defines are converted
to macro parameters for the memcpy_simple variants.

Change the benchmark program with the first three tests trying to more
closely mimic real-world memcpy usage. The tests are aligned copies with
size of a power of two up to 4096, aligned copies with size of a multiple
of four up to 1024, and unaligned copies of sizes up to 1023.
Add preload catch up for line size of 64 in simplified memcpy

Implement optional preload catch up for line size 64 for the word
aligned case. It doesn't seem to provide a performance benefit however.