Ninja: XL Fortran preprocessing with -E

In !4075 (merged) and !4735 (merged) we implemented support for XL Fortran with the Ninja generator. This required a special cpp helper script to work around the lack of any xlf option to explicitly preprocess without also compiling.

#19450 (comment 658510) mentioned that XL Fortran is gaining support for explicit preprocessing with the -qpreprocess -E options. This has now been documented by IBM here:

The -E option is added to XL Fortran starting from XL Fortran for Linux, V16.1.1.7.

This means that, at least on Linux, we could switch to using that option instead of the special script when the compiler is sufficiently new.