This adds -fwasm-exceptions (in similar fashion with
-fdwarf-exceptions or -fsjlj-exceptions) that turns on everything
with wasm exception handling from the frontend to the backend.
We currently have -mexception-handling in clang frontend, but this is
only about the architecture capability and does not turn on other
necessary options such as the exception model in the backend. (This can
be turned on with llc -exception-model=wasm, but llc is not invoked
separately as a command line tool, so this option has to be transferred
Turning on -fwasm-exceptions in clang also turns on
-mexception-handling if not specified, and will error out if
-mno-exception-handling is specified.