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
Brad King