[dev.cc] runtime: bring back mgc0.h
[dev.cc] runtime: bring back mgc0.h

This was recorded as an hg mv instead of an hg cp.
For now a C version is needed for the Go compiler.

TBR=r
CC=golang-codereviews
https://codereview.appspot.com/174020043

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