Lower intrinsic instruction "llvm.trap" to a call of the abort() function.

Hi Cristian,

This is the patch for handling `llvm.trap`.

`llvm.trap`
> is lowered to the target dependent trap instruction. If the target does not have a trap instruction, this intrinsic will be lowered to a call of the abort() function.

from LLVM LangRef.

Since we don't do the target dependent translation part, I directly lower it to a call of the `abort()` function.

And since this is my first time writing some code using LLVM, your review is highly appreciated. Please check the correctness, etc. Thanks!