JS: add support for linking C sources

Support linking C sources with JS output of the JavaScript backend.
See the added documentation in the users guide.

The implementation simply extends the JS linker to use the objects (.o)
that were already produced by the emcc compiler and which were filtered
out previously. I've also added some options to control the link with C
functions (see the documentation about pragmas).

With this change I've successfully compiled the direct-sqlite package
which embeds the sqlite.c database code. Some wrappers are still
required (see the documentation about wrappers) but everything generic
enough to be reused for other libraries have been integrated into
rts/js/mem.js.