[wasm] EmccCompile: Improve AOT time by better utilizing the cores (#67195)

[wasm] EmccCompile: Improve AOT time by better utilizing the cores (#67195)

* [wasm] EmccCompile: Improve AOT time by better utilizing the cores

Problem:

`EmccCompile` tasks compiles `.bc` files to `.o` files, and uses
`Parallel.ForEach` to run `emcc` for these in parallel.

The problem manifests when `EmccCompile` is compiling lot of files.
- To start with, the intended number of cores are being used
- but at some point (in my case after ~150 out of 180 files), the number
  of cores being utilized goes down to 1.
- And the reason is that `Parallel.ForEach` partitions the list of
  files(jobs), and they execute only the assigned jobs

From: https://github.com/dotnet/runtime/issues/46146#issuecomment-754021690

Stephen Toub:
    "As such, by default ForEach works on a scheme whereby each
    thread takes one item each time it goes back to the enumerator,
    and then after a few times of this upgrades to taking two items
    each time it goes back to the enumerator, and then four, and
    then eight, and so on. This ammortizes the cost of taking and
    releasing the lock across multiple items, while still enabling
    parallelization for enumerables containing just a few items. It
    does, however, mean that if you've got a case where the body
    takes a really long time and the work for every item is
    heterogeneous, you can end up with an imbalance."

The above means that with wildy different times taken by each job, we
can end up in this imbalance, leading to some cores being idle, which
others get reduced to running jobs sequentially.

Instead, we want to use work-stealing so jobs can be run by any partition.

In my highly unscientific testing, with AOT for `System.Buffers.Tests`,
the total time to run `EmccCompile` for 181 assemblies goes from 5.7mins
to 4.0mins .

* MonoAOTCompiler.cs: Ensure that the parallel jobs get scheduled with

.. work-stealing, instead of being partitioned.