It fails this way for all fortran programs.
Could you post a minimal example of what's not working?
For me it works (in released 12.0), if I add -I/usr/local/flang/include to the compilation calls to flang, and add -lexecinfo to linking calls to flang.
(IMHO the need to add these extras is a bug in itself)
I've just made a major update to flang (and dependencies). With it, lexecinfo and -I/usr/local/flang/include are not necessary any longer. OpenMP pragmas seem to work out of the box.
Please reopen if this is still an issue.