With this CL it is possible to compile a wasm module with multiple
threads in parallel. Parallel compilation works as follows:

1)   The main thread allocates a compilation unit for each wasm function.
2)   The main thread spawns WasmCompilationTasks which run on the
     background threads.
3.a) The background threads and the main thread pick one compilation unit
     at a time and execute the parallel phase of the compilation unit.
     After finishing the execution of the parallel phase, the compilation
     unit is stored in a result queue.
3.b) If the result queue contains a compilation unit, the main thread
     dequeues it and finishes its compilation.
4)   After the execution of the parallel phase of all compilation units has
     started, the main thread waits for all WasmCompilationTasks to finish.
5)   The main thread finalizes the compilation of the module.

I'm going to add some additional tests before committing this CL.

R=titzer@chromium.org, bmeurer@chromium.org, mlippautz@chromium.org, mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/1961973002
Cr-Commit-Position: refs/heads/master@{#36178}