runtime: fix nacl/amd64p32 build
runtime: fix nacl/amd64p32 build

BP is not a legal register on nacl.

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/140980043

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