Rename cgroup.cpp to cgroupcpu.cpp

[am11]

In this case, two objects (.o) with same name are linked in a static library (gc/unix/cgroup.cpp and nativeaot/Runtime/unix/cgroup.cpp) and the linkage order of objects is nondeterministic. When such a static library is linked into the final object on macOS (executable or library), and we run dsymutil on it to strip its symbols, dsymutil does not look past the first object by name and intermittently warns about missing symbols when the linkage order was reversed. That warning turns into error in NativeAOT publishing process.

Simplest fix is to rename the source file to disambiguate the output object filename.

Also refactor the code a bit: instead of using header for InitializeCpuCGroup and extern for GetCpuLimit, use header for both APIs.

Revert #82881
Fix #80934

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Issue Details

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• Instead of using header for InitializeCpuCGroup and extern for GetCpuLimit, use header for both APIs.
• Remove CGroup class with static methods and replace with flat functions defined in order.

This prevents dsymutil on macOS intermittent failures to find these symbols off of static libs.

Revert #82881
Fix #80934

Author:	am11
Assignees:	-
Labels:	area-NativeAOT-coreclr
Milestone:	-

[am11]

@jkotas, I ran build in 50x loop on M1 machine and found no failure (it fails about every third build without the second commit). Filename cgroupcpu.cpp is chosen based on its existing header cgroupcpu.h.

[jkotas]

Thank you!