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

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go