Disable the emergency CPU limit on HHVM

The safety assumptions aren't valid there -- HHVM does not handle
max_execution_time with a longjmp() from a signal handler. In fact, its
use of a separate stack for JITed code causes longjmp() to abort with an
error if you try to call it from a signal handler.

Change-Id: I750112601865b3b61d5676649e08579e73839c06