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

We changed cgo to write the actual function wrappers in Go
for Go 1.4. The only code left in C output files was the definitions
for pointers to C data and the #pragma cgo directives.
Write both of those to Go outputs instead, using the new
compiler directives introduced in CL 169360043.

(Still generating C files in gccgo mode.)

LGTM=r
R=r
CC=austin, golang-codereviews, iant, khr
https://golang.org/cl/169330045