runtime: use uint32 arrays in BenchmarkClearFat*
runtime: use uint32 arrays in BenchmarkClearFat*

The CopyFat benchmarks were changed in CL 92760044. See CL 111350043 for discussion.

LGTM=khr
R=khr
CC=golang-codereviews
https://codereview.appspot.com/116000043

d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh