XL: Install our Fortran 'cpp' helper script with execute permission

This script was added by commit 19f267c75e (XL: Add support for Ninja
and XL Fortran, 2019-11-21, v3.17.0-rc1~368^2) but does not have a `.sh`
extension so our existing install rules neglect to give it execute
permission.  Our test suite works on XL Fortran in the build tree but
the script is broken without execute permission on installation.

Fixes: #20695

CMakeLists.txt

@@ -810,7 +810,7 @@ if(NOT CMake_TEST_EXTERNAL_CMAKE)
     PATTERN "*.sh*" PERMISSIONS OWNER_READ OWNER_EXECUTE OWNER_WRITE
                                 GROUP_READ GROUP_EXECUTE
                                 WORLD_READ WORLD_EXECUTE
-    PATTERN "ExportImportList"
+    REGEX "/(ExportImportList|cpp)$"
       PERMISSIONS OWNER_READ OWNER_EXECUTE OWNER_WRITE
                   GROUP_READ GROUP_EXECUTE
                   WORLD_READ WORLD_EXECUTE