[SjLj] __USING_WASM_SJLJ__ -> __WASM_SJLJ__

[aheejin]

In line with #21970, this changes __USING_WASM_SJLJ__ to __WASM_SJLJ__. We don't define this in Clang now (which we can maybe consider, but given that Clang doesn't have dedicated flags for SjLj and there don't seem to be other platforms doing similar things), but given that the affected files currently are only in Emscripten, I think we can safely change this.

[sbc100]

Do we need this macro in practice? Can we perhaps merge it with WASM_EXCEPTIONS?

[aheejin]

That's a good question... I think we separated it mainly because there was time we supported Wasm EH but not Wasm SjLj so we let people use Wasm EH with Emscripten SjLj. This still works but not the recommended mode of using it anymore; we now use Wasm SjLj by default if -fwasm-exceptions is given. But you can still manually override it by -fwasm-exceptions -sSUPPORT_LONGJMP=emscripten or something.

Merging the two preprocessor means we disallow using Wasm EH with Emscripten SjLj. Would that be OK? (At this point I can't think of a reason why someone wants to use Wasm EH with Emscripten SjLj though)

[sbc100]

That's a good question... I think we separated it mainly because there was time we supported Wasm EH but not Wasm SjLj so we let people use Wasm EH with Emscripten SjLj. This still works but not the recommended mode of using it anymore; we now use Wasm SjLj by default if -fwasm-exceptions is given. But you can still manually override it by -fwasm-exceptions -sSUPPORT_LONGJMP=emscripten or something.

Merging the two preprocessor means we disallow using Wasm EH with Emscripten SjLj. Would that be OK? (At this point I can't think of a reason why someone wants to use Wasm EH with Emscripten SjLj though)

The more configurations we can disallow the better IMO!