* functional mac port
* combine common parts of Linux and Darwin implementations
* add unwind support for apple
* insure BACKWARD_HAS_BACKTRACE_SYMBOL is set for BACKWARD_SYSTEM_DARWIN
* fix indentation
* use pthread_self and pthread_main_np
*Read the pc as specified for aarch64 processors.
*Disable division by zero tests, since aarch64 does not trap these and
therefore does not generate SIGFPEs there.
*All other tests succeed.