Rust upgrade to LLVM6 breaks wee_alloc

[Mirclus]

wee_alloc fails to build with the latest nightly (and possibly yesterdays as well) and most likely will continue to fail with upcoming versions (target: wasm32-unknown-unknown):

Intrinsic has incorrect return type!
void (i32)* @llvm.wasm.grow.memory.i32
LLVM ERROR: Broken function found, compilation aborted!

This is most likely caused by the upgrade to LLVM 6 (rust-lang/rust#47828), which changed the return type of this intrinsic.

[fitzgen]

Thanks foir the heads up! Should be a pretty easy fix if you're interested in contributing. Just need to change the return type in imp_wam32.rs and make the alloc_pages function take advantage of the return type.