[Isolates] Use _exit() instead of exit() in shell to avoid shutdown

[Isolates] Use _exit() instead of exit() in shell to avoid shutdown
races between static descructors and isolate threads.

Our tests that throw exceptions used to call exit() in the thread that
threw the exception. This invoked static descructors while the other
threads were still running and we had all kinds of bad effects.

Committed: http://code.google.com/p/v8/source/detail?r=7090

[Dmitry Titov, 2011-03-07 23:32:03]

LGTM