go: cmd/go: cgo builds don't work with "zig cc"

Hello, I am filing this issue on behalf of an issue reported upstream here: https://github.com/ziglang/zig/issues/7342

@lu4p writes:

---

I would like to use “zig cc” for easily cross compiling cgo dependent go code, however the “drop-in” c compiler replacement doesn’t work.

Example cgo code

main.go

package main

//int Add(int a, int b){
//    return a+b;
//}
import "C"
import "fmt"

func main() {
	a := C.int(10)
	b := C.int(20)
	c := C.Add(a, b)
	fmt.Println(c) // 30
}

other compilers

go build main.go //build works fine (uses gcc)
CC="clang" go build main.go //build works fine (uses clang)

zig cc

$ CC="zig cc" go build main.go
# runtime/cgo
info: Usage: zig [command] [options]

Commands:

  build            Build project from build.zig
  build-exe        Create executable from source or object files
  build-lib        Create library from source or object files
  build-obj        Create object from source or assembly
  cc               Use Zig as a drop-in C compiler
  c++              Use Zig as a drop-in C++ compiler
  env              Print lib path, std path, compiler id and version
  fmt              Parse file and render in canonical zig format
  init-exe         Initialize a `zig build` application in the cwd
  init-lib         Initialize a `zig build` library in the cwd
  libc             Display native libc paths file or validate one
  run              Create executable and run immediately
  translate-c      Convert C code to Zig code
  targets          List available compilation targets
  test             Create and run a test build
  version          Print version number and exit
  zen              Print zen of zig and exit

General Options:

  --help           Print command-specific usage

error: unknown command: -E

go version go1.15.5 linux/amd64

---

So this looks like go is inserting flags before it passes cc. The user will have to work around this by creating a shell script like this:

#!/bin/sh
zig cc $@

And then passing that as CC. This way zig gets invoked with zig cc -E ... instead of zig -E ...

This issue is a feature request for go to honor the entire command line in CC, appending flags rather than prepending them. Or, at least adding explicit detection for zig cc and passing the cc as the first arg.