benchtests: Add benches for memset with 0 value

memset with zero as the value to set is by far the majority value (99%+
for Python3 and GCC).  Add bench-memset-zero-large.c,
bench-memset-zero-walk.c and bench-memset-zero.c to measure memset
implementations for zeroing.

Reviewed-by: Sunil K Pandey <skpgkp2@gmail.com>