Fix WASM for LLVM6

[Mirclus]

Based on the implementation in dlmalloc-rs.

I'm not sure how to handle the error case. Panic? I didn't change the behavior for now.

Also, if I understand the Intrinsic correctly, both input parameter and return value should have the same type. Dlmalloc doesn't follow this either, but it uses a fixed size for the argument. I'm not really sure if this is something that should get changed.

Should fix #16.

[fitzgen]

Thanks!

We can go ahead and land this now, panicking is fine since that is what we currently do.

However, now that we can check for failure, we can also modify the alloc_pages signature so that the function returns Result<*mut u8, ()> (here and in imp_unix.rs) and properly return and propagate errors.

Interested in doing this as a follow up PR as well?