Don't try to handle nested SIGSEGV inside fault handler.

Trying to handle nested SIGSEGV in the fault handler leads to infinite signals
and retrying if the cause of the SIGSEGV was a lack of stack space. This can
lead to applications not chaining through to debuggerd and dying.
Bug: 18330119

Change-Id: Iadcffe0fa0e55d783b84a06504bbd9d181e641e4