Improve detection of CPU limits when running inside a Container

This focuses on better supporting Docker CLI's parameter `--cpus`, which limits the amount of CPU time available to the container (ex: 1.8 means 180% CPU time, ie on 2 cores 90% for each core, on 4 cores 45% on each core, etc.)

All the runtime components depending on the number of processors available are:
 - ThreadPool
 - GC
 - `Environment.ProcessorCount` via `SystemNative::GetProcessorCount`
 - `SimpleRWLock::m_spinCount`
 - `BaseDomain::m_iNumberOfProcessors` (it's used to determine the GC heap to affinitize to)

All the above components take advantage of `--cpus` via `CGroup::GetCpuLimit` with dotnet#12797, allowing to optimize performance in a container/machine with limited resources. This makes sure the runtime components makes the best use of available resources.

In the case of `Environment.ProcessorCount`, the behavior is such that passing `--cpus=1.5` on a machine with 8 processors will return `1`  as shown in https://github.com/dotnet/coreclr/issues/22302#issuecomment-459092299. This behavior is not consistent with [Windows Job Objects](https://docs.microsoft.com/en-us/windows/desktop/api/winnt/ns-winnt-jobobject_cpu_rate_control_information) which still returns the number of processors for the container/machine even if it only gets parts of the total number of cycles.

This behavior is erroneous because the container still has access to the full range of processors on the machine, and only its _processor time_ is limited. For example, in the case of a 4 processors machine, with a value of `--cpus=1.8`, there can be 4 threads running in parallel even though each thread will only get `1.8 / 8 = .45` or 45% of all cycles of each processor.

The work consist in reverting the behavior of `SystemNative::GetProcessorCount` to pre dotnet#12797.

src/classlibnative/bcltype/system.cpp

@@ -343,13 +343,6 @@ INT32 QCALLTYPE SystemNative::GetProcessorCount()
         processorCount = systemInfo.dwNumberOfProcessors;
     }
 
-#ifdef FEATURE_PAL
-    uint32_t cpuLimit;
-
-    if (PAL_GetCpuLimit(&cpuLimit) && cpuLimit < (uint32_t)processorCount)
-        processorCount = cpuLimit;
-#endif
-
     END_QCALL;
 
     return processorCount;