[wasm] Switch to the LLVM wasm backend. (#14734)

[wasm] Switch to the LLVM wasm backend. (#14734)

* [wasm] Switch to the llvm wasm backend.

* [wasm] Packager changes for the wasm backend.

* -s EMULATED_FUNCTION_POINTERS=1 no longer needed.
* Run wasm-strip on the final binary.
* Run opt during AOT.
* Disable the dynamic build for now, it doesn't seem to work with the wasm backend.

* [wasm] Use STACK_MAX instead of STACK_BASE for the bottom limit of the stack.

* [aot] Handle llvmopts= in llvmonly mode as well.

* [wasm] Disable an assertion which is no longer true with the wasm backend.

* [wasm] Disable computed gotos in the interpreter, the wasm backend doesn't support it.

Fix llvm target layout so its compatible with clang compiled code.

* [wasm] Pass llvm-path to the cross compiler.

* [wasm] Fix some problems with the dynamic build, still doesn't work. Fix packaging.

* [wasm] Pass -s DISABLE_EXCEPTION_CATCHING=0 to the runtime build, otherwise catch clauses are not generated in llvm-runtime.cpp.

* [aot] Allow llvmopts= argument multiple times.

* Bump emscripten version to 1.38.34.

* [wasm] Add https://github.com/emscripten-core/emscripten/pull/8457 as a patch.

* [wasm] Compile the aot .bc files to .o in a separate step using emcc, this will help with incremental compilation later.

* [wasm] Source emsdk_set_env.sh instead of emsdk_env.sh the latter calls emsdk construct_env to construct emsdk_set_env.sh which is racy.