[mono] limit DegreeOfParallelism to 16

[lewurm]

We started to see the System.Core-xunit step on CI to hit the timeout of 15 minutes with Linux/ARM64. That was weird, because the step used to be completed in around two minutes. With my local device (jetson board) I wasn't able to reproduce it either; it took around 100s there. We then realized it's specific to the new taishan CI machines, which are equipped with 64 cores. Hardcoding mono_cpu_count to return 16 restored the performance, however that isn't a viable fix.

Limiting DefaultDegreeOfParallelism to 16 fixes it, which is less extreme than limiting mono_cpu_count (), still not ideal though. It seems to boil down to the fact that our non-netcore threadpool implementation doesn't handle a large number of cores well.

repro.cs, extracted from here https://github.com/dotnet/corefx/blob/a9b91e205a8794327a028cb4b29953127f0f194c/src/System.Linq.Parallel/tests/QueryOperators/ConcatTests.cs#L145-L154

using System;
using System.Linq;
using System.Collections.Generic;
using System.Threading;

public class Repro {
    public static void Main (string []args) {
        const int ElementCount = 2048;
        ParallelQuery<int> leftQuery = ParallelEnumerable.Range(0, ElementCount / 4).Union(ParallelEnumerable.Range(ElementCount / 4, ElementCount / 4));
        ParallelQuery<int> rightQuery = ParallelEnumerable.Range(2 * ElementCount / 4, ElementCount / 4).Union(ParallelEnumerable.Range(3 * ElementCount / 4, ElementCount / 4));

        var results = new HashSet<int>(leftQuery.Concat(rightQuery));
        Console.WriteLine ("results.Count=" + results.Count + ", ElementCount=" + ElementCount);
    }
}

Before fix:

$ time ./mono/mini/mono-sgen repro.exe
results.Count=2048, ElementCount=2048

real    0m5.846s
user    0m0.344s
sys     0m1.929s
$ make -C mcs/class/System.Core run-xunit-test
[...]
=== TEST EXECUTION SUMMARY ===
   net_4_x_System.Core_xunit-test  Total: 48774, Errors: 0, Failed: 0, Skipped: 6, Time: 536.005s

With this fix:

$ time ./mono/mini/mono-sgen repro.exe
results.Count=2048, ElementCount=2048

real    0m1.247s
user    0m0.206s
sys     0m0.225s
$ make -C mcs/class/System.Core run-xunit-test
[...]
=== TEST EXECUTION SUMMARY ===
   net_4_x_System.Core_xunit-test  Total: 48774, Errors: 0, Failed: 0, Skipped: 6, Time: 131.143s

/cc @akoeplinger @lambdageek @filipnavara