cmd/go: provide full path as os.Args[0] when invoking tools

cmd/dist needs to re-exec or open itself to detect GOARM (CL 3973) and
detect host machine endianness (CL 14460).

Change-Id: If6438831ab0715ba8e236d64bb2c7c1bde1470aa
Reviewed-on: https://go-review.googlesource.com/14476Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/tool.go

@@ -98,6 +98,7 @@ func runTool(cmd *Command, args []string) {
 		fmt.Printf("%s\n", cmd)
 		return
 	}
+	args[0] = toolPath // in case the tool wants to re-exec itself, e.g. cmd/dist
 	toolCmd := &exec.Cmd{
 		Path:   toolPath,
 		Args:   args,