[flang] Change the suffix of the module files to be .f18.mod

so makefiles with "rm *.mod" will still work. Add a
new option to f18, -intrinsic-module-directory, which
supplies a directory in which f18 searches for standard
intrinsic modules. Neither the option nor the path is passed to F18_FC.

Original-commit: flang-compiler/f18@0bbda4c39edf170d3c194e4ad16b07e88995e46c
Reviewed-on: https://github.com/flang-compiler/f18/pull/653