Improve longjmp stubs. NFC

[sbc100]

• Use elif here so its clear that we either include one block or the next, but never both.
• Fix signature of setjmp.
• Include the _emscripten_throw_longjmp stub regardless of INCLUDE_FULL_LIBRARY settings.

This final fix is a prerequisite for #16003. Without this change LLD_REPORT_UNDEFINED results in the following different error message:

-error: longjmp support was disabled (SUPPORT_LONGJMP=0), but it is required by the code (either set SUPPORT_LONGJMP=1, or remove uses of it in the project)
+wasm-ld: error: /usr/local/google/home/sbc/dev/wasm/emscripten/cache/sysroot/lib/wasm32-emscripten/libcompiler_rt.a(emscripten_setjmp.o): undefined symbol: _emscripten_throw_longjmp