Catch signals that happen inside a fault handler.

Catch signals that happen inside a fault handler.

Unblock some signals (SIGABRT, SIGBUS, SIGSEGV) that could happen
inside of the ART internal fault handlers, to report crashes inside of
the signal handler. Because we can't use sigaction to change the
handler when this happens, because it modifies global state, add a new
member variable in Thread to track whether a call to the fault handler
is reentrant or not.

Remove the old nested signal implementation that attempted to do this.

Bug: http://b/35853436
Test: changed the #if 0 to #if 1, ran a dummy process that
      threw a NullPointerException, inspected logcat
Change-Id: I04bb4a09433c6817933d64ec681ec433b528f2a5