Fortran: Add support for submodules on Cray

Define `CMAKE_Fortran_SUBMODULE_{SEP,EXT}` for the Cray Fortran
compiler.  Use an empty separator to tell CMake that this compiler does
not use the enclosing module name as a prefix on submodule files.

Issue: #18925

Modules/Compiler/Cray-Fortran.cmake

@@ -4,6 +4,8 @@
 include(Compiler/Cray)
 __compiler_cray(Fortran)
 
+set(CMAKE_Fortran_SUBMODULE_SEP "")
+set(CMAKE_Fortran_SUBMODULE_EXT ".mod")
 set(CMAKE_Fortran_MODOUT_FLAG -em)
 set(CMAKE_Fortran_MODDIR_FLAG -J)
 set(CMAKE_Fortran_MODDIR_DEFAULT .)