[dev.cc] create new branch

This branch is for work on converting the remaining C code in
package runtime to Go and then deleting the cc, 5c, 6c, and 8c
directories. It is targeted to land at the beginning of the 1.5 cycle.

The conversion will proceed one GOOS/GOARCH combination
at a time; red lines on the dashboard are expected and okay.

Once Linux and OS X are converted, help with other systems
will be most welcome.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/174760043