Fix some things in the LLVM backend

This pull request fixes a few things in the LLVM backend.

In detail:
* it handles the case when 'to_llvm' is used but a required LLVM version (3.4 or greater) doesn't appear in $PATH,
* it fixes an error in the handling of closure names: in case of closures the actual arity is A (as of MFA) + 1, and
* it fixes a nasty error in the computation of the frame size when setting up stack descriptors.

While at it, we re-worded some comments and did a minor cleanup.