[dev.power64] cmd/gc: disable magic multiply optimizations for now
[dev.power64] cmd/gc: disable magic multiply optimizations for now

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/126110043

bryanxu/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh