Changed mono_win_chained_exception_needs_run to be a JIT TLS variable.

This fixes a potential race condition that might happen if two threads try to handle an exception at the same time.
Fix mono exception handler crashing on windows.

Here's the scenario that causes the crash:

Exception is fired, which invokes mono's vectored exception handler. It
is not an exception mono recognizes, so falls through to the default:
case in the exception handler. The problem is the return value has been
initialized to EXCEPTION_CONTINUE_EXECUTION. This causes SEH not to
process any other handlers, and just attempt to continue execution at
the point the exception was thrown.

The problem is much of windows internals will catch various exceptions
internally and handle them. So instead of the exception being handled,
we would just crash in the bowels of windows.

The solution is to set mono_win_chained_exception_needs_run to TRUE in
the default case - this is what the various exception handlers do if the
exception is not in JIT'd code. This causes the handler to return
EXCEPTION_CONTINUE_SEARCH, which causes SEH to search for the next
handler, and no crash is produced.

Both x86 and x64 had this bug, so I fixed it in both places.

Fix contributed by Steve Anichini.