[libc++abi] Don't leak in test (#119505)

Trying to re-enable a test on bots

https://github.com/llvm/llvm-zorg/blob/bb695735dba75e1a5dced13e836f4f46de464bac/zorg/buildbot/builders/sanitizers/buildbot_functions.sh#L443

When we reach `terminate()` `exc` pointer is not
on the stack, so lsan correctly reports a leak.

Author: vitalybuka

libcxxabi/test/forced_unwind2.pass.cpp

@@ -44,10 +44,8 @@ struct Stop<R (*)(Args...)> {
 };
 
 static void forced_unwind() {
-  _Unwind_Exception* exc = new _Unwind_Exception;
-  memset(&exc->exception_class, 0, sizeof(exc->exception_class));
-  exc->exception_cleanup = 0;
-  _Unwind_ForcedUnwind(exc, Stop<_Unwind_Stop_Fn>::stop, 0);
+  static _Unwind_Exception exc = {};
+  _Unwind_ForcedUnwind(&exc, Stop<_Unwind_Stop_Fn>::stop, 0);
   abort();
 }