Debug fixes.

Generate code to invoke debug_break_return rather than
debug_break_slot in BreakLocationIterator::SetDebugBreakAtReturn.

Stop emitting arm-like addresses in the instruction stream in
BreakLocationIterator::SetDebugBreakAtReturn and
BreakLocationIterator::SetDebugBreakAtSlot.

Fix RelocInfo::set_call_address and RelocInfo::call_address to
modify/retrieve the address encoded in a mov sequence.