stage1: deal with WebAssembly not supporting @returnAddress()
This makes `@returnAddress()` return 0 for WebAssembly (when not using
the Emscripten OS) and avoids trying to capture stack traces for the
general purpose allocator on that target.
1b1921f0e2Andrew Kelley committed on 8/8/2020, 8:00:29 AMparent9f5a7d5