devel/llvm-devel: build openmp as a runtime

OpenMP support can be built as a project or a runtime.  LLVM build
documentation says that when both are available, runtime is to be
prefered.  I'm making this change in advance of an expected change to
split the Flang runtime from the Flang compiler build
(https://github.com/llvm/llvm-project/pull/110217).

This changes where omp_lib.h is installed for Flang use and which
Fortran modules are installed (removes .f18.mod versions).

Sponsored by:	DARPA, AFRL