[threadpool] cache processor count

On top of mono/corefx#369 this improves the execution time of `System.Core-xunit` on Linux/ARM64 by 2x, so from:

```console
$ 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
```
to
```console
$ 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: 74.636s
```

This is only relevant for non-netcore. The CoreCLR folks just recently fixed something similar (thanks to Marek sharing this link): dotnet/coreclr#27543

mcs/class/referencesource/mscorlib/system/threading/threadpool.cs

@@ -85,7 +85,7 @@ internal static class ThreadPoolGlobals
 #if MONO
         public const uint tpQuantum = 30U;
 
-        public static int processorCount => Environment.ProcessorCount;
+        public static int processorCount = Environment.ProcessorCount;
 
         public static bool tpHosted => ThreadPool.IsThreadPoolHosted();