Support stack overflows

[msaehn]

If a application runs into a stack overflow, this package doesn't print anything.
Regarding to a message with @ianpartridge there are two issues:

• swift-backtrace listens only on SIGILL and not on SIGSEGV
• if there is an stack overflow, we need to switch to an alternate stack: https://pubs.opengroup.org/onlinepubs/009695399/functions/sigaltstack.html

Why is it important:
Because of the small stack size for secondary threads it is very easy run into a stack overflow if a lo number of recursive calls.