[flang][runtime] Support in-tree device build of Flang runtime.

I changed the set of files that are built for experimental CUDA/OMP
builds, i.e. the files with enabled device support are built
as such and the rest of the files are built just for the host target.
With this change we can build Flang runtime library that is fully functional
on the host target, so in-tree targets like check-flang become operational.

Reviewed By: klausler, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D155029