JS: support linking with C sources

Support compiling and linking C sources into JavaScript with Emscripten.

Note that we don't automatically generate wrappers. It means that linked C code isn't automatically called into. Wrappers need to provided as jsbits.

The new //#OPTIONS:EMCC=... pragma can be used to pass flags to the emcc linker. This is particularly useful to indicate which C functions to keep alive (-sEXPORTED_FUNCTIONS=...). An alternative is to use -optl.

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• fix issue with -SEXPORTED_... emcc flags not being cumulative
• add flag -fjs-c-link=auto,disabled,enabled to support the current behavior even in the presence of C files (disabled) or to force the emcc linking (enabled)
• add support for incremental linking of C files for Template Haskell using dlopen (in a future MR)
• ensure that your commits are either individually buildable or squashed
• ensure that your commit messages describe what they do (referring to tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• add a [testcase to the testsuite][adding test].
• updates the users guide if applicable
• mentions new features in the release notes for the next release