Add some files for building libxmp.js.

The build script and the wrappers to facilitate everything.
Try to disable optimisation flags.

It didn't help with the problem of fopen/etc. not found.
Correct a bug in the change of SRC_OBJS.
Add the %.show make debug incantation.
Remove the -j4 for easier debugging.
Fix some bugs in the function exporting.

Thanks to azakai from #emscrpiten on irc.mozilla.org.
Add another required wrapper.

For xmp_js__sizeof.