The `Platform/<OS>.cmake` modules record platform-specific flag
variables for the C toolchain.  The `CMake<LANG>Information` modules
initialize the per-language platform-specific flag variables to defaults
copied from C.  However, they have diverged over time, and not all
platform-specific flag variables are initialized correctly for all
languages.  Factor out the CXX language's instance of this logic into a
helper macro, and reuse it for all languages for which we previously
had the platform-specific flag variables at least partially filled out.

Previously the Fortran language's copy of this logic used
`if(NOT DEFINED <var>)` instead of just `if(NOT <var>)` to allow
`Platform/<OS>-<COMPILER_ID>-<LANG>` modules to specify empty values for
some platform-specific flag variables without being overridden.
Use this approach in the helper macro so it applies to all languages.

Fixes: #25990