runtime: add Benchmark[Clear|Copy]Fat[8|12]
runtime: add Benchmark[Clear|Copy]Fat[8|12]

These correspond to 2 and 3 word fat copies/clears on 8g, which dominate usage in the stdlib. (70% of copies and 46% of clears are for 2 or 3 words.) I missed these in CL 111350043, which added 2 and 3 word benchmarks for 6g. A follow-up CL will optimize these cases.

LGTM=khr
R=khr
CC=golang-codereviews
https://codereview.appspot.com/115160043

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