src/os: add stubs for exec.ExitError and ProcessState.ExitCode

This commit adds stubs for the `ExitError` struct of the `exec` package
and `ProcessState.ExitCode()` of the `os` package.

Since the `os/exec` is listed as unsupported, stubbing these methods
and structs should be enough to get programs that use these to compile.

src/os/exec.go

@@ -34,6 +34,12 @@ func (p *ProcessState) Success() bool {
 	return false // TODO
 }
 
+// ExitCode returns the exit code of the exited process, or -1
+// if the process hasn't exited or was terminated by a signal.
+func (p *ProcessState) ExitCode() int {
+	return -1 // TODO
+}
+
 type Process struct {
 	Pid int
 }