Set CMAKE_SYSTEM_NAME to iOS, tvOS, watchOS from CMake 3.14

[czoido]

Changelog: Fix: Set CMAKE_SYSTEM_NAME set to iOS, tvOS or watchOS or Darwin depending on the CMake version.
Docs: Omit

Closes: #7875

Since CMake 3.14 setting CMAKE_SYSTEM_NAME to iOS, tvOS or watchOS is supported. This PR checks the version of CMake in the build helper and sets the definition accordingly.

TODO: Update this implementation in the toolchain files. I'll open an issue about this.

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