[libc++][Android] Include cstdlib to define __BIONIC__

[rprichard]

Summary

ANDROID is a built-in compiler macro, but BIONIC is defined by
the libc header.

[compnerd]

Seems like a straightforward change for the tests to repair them on Android.

libcxxabi/test/forced_unwind4.pass.cpp

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: linux && target={{aarch64-.+}}
 
 // pthread_cancel in case of glibc calls _Unwind_ForcedUnwind from a signal on
 // the child_thread. This test ensures sigretrun is handled correctly (see:
 // UnwindCursor<A, R>::setInfoForSigReturn).
 
+#include <cstdlib> // defines __BIONIC__
+
 // Android/Bionic does not support pthread_cancel.
 #ifdef __BIONIC__
 int main() {
   return 0;
 }
 #else
 
 #include <chrono>