'Hello world' crystal shared library experiment

This is a baby experiment on LLVM compilation, to build a shared library from crystal source code

Simple Crystal code, with an empty prelude

Description

The generated library will exit the current process with error code 42.

Run the test

$ cd ./simple_no_prelude

1. Build the tester run_shared (written in C)

   $ make run_shared

2. Build the shared library hello_llvm.so (written in Crystal, without stdlib)

   $ make hello_llvm.so

3. Finally, run the binary run_shared, this will open the hello_llvm.so shared library, locate a function named crystal_do_something, then run it.

   $ ./run_shared
   $ echo $?
   42