FindMPI: Fix finding IntelLLVM wrappers on Windows

In commit 5e700411d2 (FindMPI: add IntelLLVM MPI wrappers, 2024-01-19,
v3.29.0-rc1~92^2) we added `mpiicpx.bat` for C++ on Windows, but that is
a GNU-like front-end that we do not yet support.  Use `mpiicx.bat` for
both C and C++ on Windows, just as we use `icx` to compile both.

Fixes: #25807