Bugfix: Support old installations using Cray MPICH (#20663)
Bugfix: Support old installations using Cray MPICH (#20663)

#20076 moved Cray-specific MPICH support from the Spack MPICH package
to a new cray-mpich Package. This broke existing package installs
using external mpich on Cray systems. This PR keeps the cray-mpich
package but restores the Cray-specific MPICH support for older
installations.

In the future this support should be removed from the Spack mpich
package and users should be directed to use cray-mpich on Cray.
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack