Redirect calls to printf and fprintf

[hahnjo]

When the interpreted code calls an external function, LLVM will use
dlsym() to find the target in the current process. This works well
for functions from libc which are always loaded.
However this also outputs the result of printf() and fprintf() to the
terminal and does not forward it to the user. This is very unintuitive
because values piped into std::cout and std::cerr correctly appear in
the browser.

As a solution, use llvm::sys::DynamicLibrary::AddSymbol() to inject
custom implementations into the process of finding external functions.
Once hooked up, the implementations format the passed arguments into
a C++ string and forward the result to the C++ streams that correctly
forward to the user.

Fixes #112

[SylvainCorlay]

This is great!

[JohanMabille]

Thanks a lot!

[SimeonEhrig]

@hahnjo
Sorry for the delay. I was very busy last week. Great Work!

From PR #293

As far as I know, it's undefined behavior to redefine standard functions like printf. Instead I've independently developed a different solution in #315 that injects custom implementations (very similar to this PR) into the process of calling external functions from interpreted code.

Yes, it's similar to my idea, but you solved my last problem, the os dependency, in a really clever way.

I have also a possible improvement for your implementation. When, I use fprintf(stdout, "Hello World!\n"); it prints to the terminal, but I would expect, that it prints to the browser. It's just a small fix.

Thank you very much for your work. It helps me a lot with our old school developers ;-)

[hahnjo]

@SimeonEhrig

I have also a possible improvement for your implementation. When, I use fprintf(stdout, "Hello World!\n"); it prints to the terminal, but I would expect, that it prints to the browser. It's just a small fix.

Yes, the code in fprintf_jit should likely handle the case of stream == stdout, good thought.

[hahnjo]

I have also a possible improvement for your implementation. When, I use fprintf(stdout, "Hello World!\n"); it prints to the terminal, but I would expect, that it prints to the browser. It's just a small fix.

Yes, the code in fprintf_jit should likely handle the case of stream == stdout, good thought.

See #320 for the obvious follow-up.