The Ninja generator's support for Fortran requires that source files
be preprocessed explicitly first.  However, the `xlf` compiler does
not have a simple `-E` option or equivalent to do preprocessing.
The only documented way to get preprocessed output is to use `-d`
to leave it behind, but only at an inflexible location.

Instead, create our own `cpp` wrapper script and substitute it for the
real preprocessor using `-tF -B ...`.  Teach the wrapper to map the
`cpp` output to the location we need and then invoke the real `cpp`
underneath.

Fixes: #19450