Should be OK for mingw.org's MinGW. I cannot test the MinGW64 bits, but I trust that you did.

The attached patch implements getting the number of processors on MS-Windows.

Indeed, different versions of MinGW use different symbols to guard the declaration, and using those symbols in not future-proof enough, IME.
Removing the declaration is definitely the best solution.

Yes, I see the problem:

In T5889#156302, @gniibe wrote:

Considering again, I think that just removing the definition of the struct timespec in npth.h is the best approach, given the situation, it's been there for MINGW64 and it's now in original MinGW.

Let me ask a more specific question, since you mentioned "support of detecting numbers of CPU and having more than 1 CPUs": can you point me to the code which detects the number of CPUs on MS-Windows systems, where I could learn how that code is affected by having EOPNOTSUPP defined? I will then hopefully understand better what you are saying, and either agree with you that this is unworkable on Windows, or propose a better solution.

I guess I don't understand what you mean by "native building". This build was with MinGW, which is as "native" as MinGW64 is.
The reason for the problem is (AFAIU) that MinGW64 went after Microsoft's change in stat due to the 32-bit vs 64-bit time and off_t values issue.s That change breaks backward compatibility in more than one way: programs compiled on some versions of Windows will not run on other versions. mingw.org's MinGW kept the original semantics and symbols, which is why _stat32 exists in the mingw.org's headers, but is not exposed by default.

Can you please tell more about how this causes non-working code? MinGW64 defines EOPNOTSUPP to an arbitrary constant which (AFAICT) is never actually returned or used in the MS-Windows runtime. Their documentation, in https://docs.microsoft.com/en-us/cpp/c-runtime-library/errno-constants?view=msvc-170, says:

This is with mingw.org's MinGW, version 5.4.x.

The version of MinGW is 5.4.x, the latest one. It is available from https://osdn.net/projects/mingw/releases.
MinGW64 is a fork of the above (original) MinGW. They have unfortunately diverged, thus the need to have these changes.