gnu: openmpi: Reintroduce PSM (Intel TrueScale) support.

Fixes a regression introduced in
9129804230a3a4272731609f6418b876ba450575 whereby PSM would never be
added as an input.

* gnu/packages/mpi.scm (openmpi)[inputs]: Re-add PSM on supported
systems.

gnu/packages/mpi.scm

@@ -168,10 +168,10 @@ bind processes, and much more.")
      `(("hwloc" ,hwloc "lib")
        ("gfortran" ,gfortran)
        ("libfabric" ,libfabric)
-       ,@(match (%current-system)
-                ((member (package-supported-systems psm))
-                 `(("psm" ,psm)))
-                (_ `()))
+       ,@(if (and (not (%current-target-system))
+                  (member (%current-system) (package-supported-systems psm)))
+             `(("psm" ,psm))
+             '())
        ("rdma-core" ,rdma-core)
        ("valgrind" ,valgrind)))
     (native-inputs