The default executable extension for wasm should be .wasm

Currently, when the output executable name is not specified at link time, the GHC wasm backend produces a file without any extension. Following the convention in the wasm ecosystem, the default file extension should be .wasm instead.

It's very easy to change, but note that Cabal needs to be changed as well:

• https://github.com/haskell/cabal/blob/master/Cabal/src/Distribution/Simple/Utils.hs#L1535
• https://github.com/haskell/cabal/blob/master/Cabal/src/Distribution/Simple/BuildPaths.hs#L235