There are many reports that sigaltstack() on Apple platforms stops
backtrace() from tracing an original stack, e.g.

* https://github.com/llvm/llvm-project/commit/d39ead41056b00d10b6223543562cd304307ac9b
* https://reviews.llvm.org/D28265

If we disable an alternate stack for signal handler, we won't be able to
catch stack overflow errors, but backrace() will work again.