std: Synchronize global allocator on wasm32
std: Synchronize global allocator on wasm32

We originally didn't have threads, and now we're starting to add them!
Make sure we properly synchronize access to dlmalloc when the `atomics`
feature is enabled for `wasm32-unknown-unknown`.

GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust