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.