FindMPI: Interrogate Cray compiler

Cray traditionally does not ship mpicc/mpic++ compiler wrappers.
Due to that, the traditional CMake logic to find MPI is to *only*
support MPI on Cray systems if either

1. Cray Wrappers are used for compiling, or
2. an auxiliary MPI executable is found.

On many new Exascale prototypes, using the Cray wrappers for develop
work is extremely painful. Vendors such as AMD and integrators such
as HPE seem not to go well in lock-step in Cray programming envs,
making offloading with Cray wrappers too challenging. On the other
hand, using the working experimental Vendor compilers works well,
as long as one does not need MPI.

Extend the fallback MPI search logic by also interrogating the
CC/cc/ftn binaries if found for their MPI flags. This does not
change existing logic, i.e. using the Cray wrappers directly, but
provides an additional way to get MPI programs compiled on Cray.

Fixes: #22368

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