cmd/interal/ld: darwin c-archive buildmode support

Uses ar to create an archive when -buildmode=c-archive.

A small example (that I hope to turn into a test in a later CL):

goarchive.go:
	package main

	import "fmt"

	import "C"

	func init() {
		fmt.Println("ran go init")
	}

	//export FuncInGo
	func FuncInGo() {
		fmt.Println("called a go function")
	}

	func main() {
		fmt.Println("in main")
	}

This can be compiled with:

	go build -ldflags=-buildmode=c-archive -o=libgo.a goarchive.go

main.c:

	#include <stdio.h>

	extern void FuncInGo();

	int main(void) {
		printf("c hello\n");
		FuncInGo();
		printf("c goodbye\n");
		return 0;
	}

Can be compiled with:

	cc main.c libgo.a

Apple provide a warning about the lack of PIE, but still produce a
binary which runs and outputs (on darwin/amd64):

	c hello
	ran go init
	called a go function
	c goodbye

Change-Id: I7611925f210a83afa6bd1e66a5601dd636a428c8
Reviewed-on: https://go-review.googlesource.com/8711
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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