Compiler arguments coming from CC environment variables or multi-element
CMAKE_<LANG>_COMPILER variables set by toolchain files were previously not
exposed in the file API. Among other possible problems, this caused clients
to determine wrong system include paths and built-in preprocessor macros by
calling the compiler without these important arguments.
This is fixed by adding an optional "commandFragment" attribute to the
compiler description in the `toolchains` object, containing these arguments
as a command line fragment. This is already the form in which they are
internally stored in the CMAKE_<LANG>_COMPILER_ARG1 variable, so all that is
required is adding this variable to the set of exported variables, besides
some logic to omit it if empty.
Issue: #22568
