#122466 had an unexpected side effect that,
`EnableFeaturesForWasmEHSjLj` and `BanIncompatibleOptionsForWasmEHSjLj`
can be called multiple times now, every time a Wasm EH flag
(`-fwasm-exceptions`, `-wasm-enable-eh`, `-wasm-enable-sjlj`, ..) was
checked and handled. This resulted in unnecessarily adding the same
feature-enabling arguments multiple times to the command line, for
example, `-target-feature +exception-handling` could be added as many as
three times, which didn't cause any errors but unnecessary. Also we ran
`BanIncompatibleOptionsForWasmEHSjLj` more than once, which was harmless
but unnecessary.
This guards these functions with a static variable so that we only run
them once.
25825d4ac9