This tries to make Wasm compilable in LLVM tree with CMake for
non-Emscripten platform.

This
- Adds `-D__USING_WASM_EXCEPTIONS__` when you compile with
  `-fwasm-exceptions` (like other EH options) in Clang
- Exclude `UnwindLevel1.c` when compiling with Wasm
- Changed some `__USING_WASM_EXCEPTIONS__` to `__wasm__`; they should be
  applied when compiling with Wasm w/o exceptions.
- Define some unused macros to make it compile