CMake: [skip ci] allow empty CMAKE_SYSTEM_PROCESSOR

[looooo]

Thank you for creating a pull request to contribute to FreeCAD! To ease integration, please confirm the following:

• Branch rebased on latest master git pull --rebase upstream master
• Unit tests confirmed to pass by running ./bin/FreeCAD --run-test 0
• Commit message is well-written
• Commit message includes issue #<id> or fixes #<id> where <id> is the associated MantisBT issue id if one exists

And please remember to update the Wiki with the features added or changed once this PR is merged.
Note: If you don't have wiki access, then please mention your contribution on the 0.19 Changelog Forum Thread.

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[wwmayer]

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