[This CL is part of the removal of C code from package runtime.
See golang.org/s/dev.cc for an overview.]

Make gcToolchain.cc return an error (no C compiler!).

Adjust expectations of cgo, now that cgo does not write any C files
(no C compiler!).

For packages with .s files, invoke Go compiler with -asmhdr go_asm.h
so that assembly files can use it. This applies to all packages but is only
needed today by package runtime.

LGTM=r
R=r
CC=austin, golang-codereviews, iant, khr
https://golang.org/cl/171470043