Initialize barrier_cache for ARM EH ABI compliance

Initialize barrier_cache for ARM EH ABI compliance

The ARM Exception Handling ABI requires personality functions in
phase1 to initialize barrier_cache before returning
_URC_HANDLER_FOUND, and we don't.

Although our own ARM personality function does not use barrier_cache
at all, other languages' ARM personality functions, during phase2, are
allowed and expected to test barrier_cache.sp to check whether the
handler frame was reached, which implies that personality functions is
in charge of the frame, and the remaining fields of barrier_cache hold
whatever values it put there in phase1.

Since we did not set barrier_cache.sp, an earlier exception, already
handled by a non-Ada handler and then released, may have its storage
reused for a new exception, that phase1 matches to an Ada frame, but
if that leaves barrier_cache.sp alone, the phase2 personality function
that handled the earlier exception, upon reaching the frame that
handled the earlier exception, may believe the information in
barrier_cache applies to the current exception.  The C++ personality
function, for example, would take the information in the barrier_cache
and end up activating the handler that handled the earlier exception:

  try {
    throw 1;
  } catch (int i) {
    std::cout << "caught " << i << " by c++" << std::endl;
  }
  raise_ada_exception (); // might loop back to the handler above

for  gcc/ada/ChangeLog

	* raise-gcc.c (personality_body) [__ARM_EABI_UNWINDER__]:
	Initialize barrier_cache.sp when ending phase1.