cmake-toolchain: set CMAKE_SYSTEM_PROCESSOR in Apple block

[theodelrieu]

Hello,

CMAKE_SYSTEM_PROCESSOR is not set by CMakeToolchain when building on Apple, I'm not sure if this is the right place to set it.

It is mentioned in GenericSystemBlock, but after debugging a bit, the values returned by self._get_cross_build() are all None.

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

[memsharded]

Are you sure that using different architecture for host and build profiles it was not defined? I am concerned that this can conflict with the GenericSystemBlock one in some cases.

Also a bit more detailed why/when this is needed, problems that it is solving, etc. So far we have apple OSX, iOS and M1 cross-builds in our testing, and all seems to be working fine.

It would also be needed at least some unit testing. We can help with that once the problem and scope is clear.

[theodelrieu]

Also a bit more detailed why/when this is needed, problems that it is solving, etc.

We have a library that checks CMAKE_SYSTEM_PROCESSOR to determine if thread_local should be used or not (CMake has cxx_feature stuff, but thread_local detection is buggy).

When cross-building for 32-bit x86 simulator, the library did define thread_local, which is not supported on this platform. I realised that CMAKE_SYSTEM_PROCESSOR was empty.

I am concerned that this can conflict with the GenericSystemBlock one in some cases.

True, I guess finding out why _get_cross_build returns empty values would be a cleaner way to tackle the problem 😅

[memsharded]

Superseded by #10856, which is targeted for next 1.47.
Thanks very much for raising this!