Allow multiple entry points in a single binary

[dzagieboylo]

From @gharrma on May 15, 2018 14:39

Right now there is a PolyLLVM flag that chooses which entry point (i.e., main function) to use.

Normally, though, users expect to be able to choose an entry point when launching a Java program. We could support this by using the first command line argument (of the compiled program) to choose the correct entry point. Using dlsym might even make this easy.

Copied from original issue: gharrma/polyllvm#53