MFC r323997-r323998

r323997:
Fix handling of uncaught exceptions in a std::terminate() handler on arm.

When raising an exception, the unwinder searches for a catch handler and if
none is found it should invoke std::terminate() with the uncaught exception
as the "current" exception. Before this change, the terminate handler was
invoked with no exception as current (abi::__cxa_current_exception_type()
returned NULL), because the return value from the unwinder indicated an
internal failure in unwinding. It turns out that was because all errors
from get_eit_entry() were translated to _URC_FAILURE. Now the error is
returned untranslated, which allows _URC_END_OF_STACK to percolate upwards
to throw_exception() in libcxxrt. When it sees that return status it
properly calls std::terminate() with the uncaught exception installed
as the current exception, allowing custom terminate handlers to work
with it.

r323998:
Fix the return value from _Unwind_Backtrace() on arm.

If unwinding stops due to hitting the end of the call chain, the return
value is supposed to be _URC_END_OF_STACK; other values indicate internal
errors. The return value from get_eit_entry() is now returned without
translating it to _URC_FAILURE, so that callers can see _URC_END_OF_STACK
when it happens.