Refactor exclusion of -I/usr/include to avoid per-language values
Add a `CMAKE_PLATFORM_IMPLICIT_INCLUDE_DIRECTORIES` to contain the hard-coded list of paths to be excluded from `-I` arguments so that the values remain excluded even if the per-language `CMAKE_<LANG>_IMPLICIT_INCLUDE_DIRECTORIES` variants change. This is needed to preserve our historical exclusion of `-I/usr/include` even when it is not a real implicit include directory. A policy may be needed to remove it later.
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