cmd/go: prohibit C sources files unless using cgo

Those C files would have been compiled with 6c.
It's close to impossible to use C correctly anymore,
and the C compilers are going away eventually.
Make them unavailable now.

go1.4.txt change in CL 145890046

LGTM=iant
R=iant
CC=golang-codereviews, r
https://codereview.appspot.com/149720043
doc/go1.4.txt: document that C sources in non-cgo packages are no longer allowed

CC=golang-codereviews
https://codereview.appspot.com/145890046