Check for required LLVM version or issue error

This checks that a required LLVM version (i.e. 3.4 or greater) appears in $PATH
when 'to_llvm' flag is used; in case of failure, abort compilation with an
error.
Fix counting of arguments of closures

Do not rely on MFA name for the arity of functions, since closures have
an extra argument. Instead, just use the length of the arguments list.
Fix frame size adjustment of stack descriptors

In case of function calls with arguments that are passed to the stack,
the frame size of corresponding stack descriptors needs to be reduced by
the number of stack arguments. This commit fixes a bug in this
adjustment which was caused by an incorrect check.