Look at simplifying build instructions to depend on distro Emscripten packages

[rib]

One of the thorns that makes it painful for people to fetch, build and test GPU Top is that Emscripten depends on a special branch of llvm and the build instructions currently describe building the llvm-fastcomp branch from source instead of depending on distro Emscripten packages.

At least in the past we found the Ubuntu package for Emscripten wasn't able to build gputop-web.c correctly, but it could be that more recent packages work fine.

If we can verify that the web ui built on Ubuntu 14.04 using the emscripten package works then we should instead remove the instructions to build llvm from source from the wiki Build Instructions.

[rib]

I did look into this again but Ubuntu's Emscripten packages aren't usable