Add IBM XL Fortran compile command to fpm_compiler_arguments function

[rouson]

This PR ensures that the check_specification_expression() function in the compiler_test_m module recognizes and is able to invoke the IBM XL Fortran compile using execute_command_line subroutine, which is required because the only standard-conforming way to observe type finalization in a specification expression is through error termination (because the final subroutine must be pure and therefore can't do I/O).