Backport r39378 to fix at_exit hooks with rescue behavior.

```
Fri Feb 22 14:28:17 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>

	* eval_jump.c (rb_exec_end_proc): remember the latest exit status.
	  [ruby-core:43173][Bug #5218]
```

ref: http://bugs.ruby-lang.org/projects/ruby-193/repository/revisions/39378/diff
Merge pull request #3 from github/fix-at_exit-behavior

Backport r39378 to fix at_exit hooks with rescue behavior.