Escape backslashes and newlines in response files (#4297)

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**What type of PR is this?**

> Uncomment one line below and remove others.
>
Bug fix

**What does this PR do? Why is it needed?**

It escapes backlashes by doubling them when using response files.
Without this, builds that trigger the usage of response files fail with:

```
<unknown line number>: internal compiler error: panic: badly formatted input
```

This comes from [this line of
code](https://cs.opensource.google/go/go/+/master:src/cmd/internal/objabi/flag.go;l=193;drc=c7ea87132f4e6f3c81e525c396a64471c9af0091;bpv=0?q=badly%20formatted%20input&ss=go%2Fgo)
in the Go compiler, which complains if a backlash is not followed either
by another `\` or by `n`.

**Which issues(s) does this PR fix?**

Fixes #4112 (I'd expect).

**Other notes for review**

I'd be happy to add a test but that function is not written in a very
testable way (and I'm not sure the response files feature as a whole has
any tests). As it is and without more significant changes I don't think
it would be possible to write a test for it.

Author: alopezz

go/tools/builders/env.go

@@ -543,7 +543,11 @@ func passLongArgsInResponseFiles(cmd *exec.Cmd) (cleanup func()) {
 	cleanup = func() { os.Remove(tf.Name()) }
 	var buf bytes.Buffer
 	for _, arg := range cmd.Args[1:] {
-		fmt.Fprintf(&buf, "%s\n", arg)
+		// Slashes need to be doubled for escaping
+		escaped_arg := strings.ReplaceAll(arg, "\\", "\\\\")
+		// Newlines too, so that they don't get split when read
+		escaped_arg = strings.ReplaceAll(escaped_arg, "\n", "\\n")
+		fmt.Fprintf(&buf, "%s\n", escaped_arg)
 	}
 	if _, err := tf.Write(buf.Bytes()); err != nil {
 		tf.Close()