[runtime] Move mono_invoke_unhandled_exception_hook to metadata

[runtime] Move mono_invoke_unhandled_exception_hook to metadata

And call it consistently in thread start functions when the managed
thread start function returns with an exception.

(If coop is not enabled, the exception handler unwinds the thread and
when there's no handler invokes the unhandled exception hook via
jit_tls->abort_func, so this is just for the coop case.)

Fixes mono/tests/unhandled-exception-1.cs for coop and bitcode.