[OpenMP][Fix] libomptarget Fortran tests (#74543)

This patch fixes the erroneous multiple-target requirement in Fortran
offloading tests. Additionally, it adds two new variables
(`test_flags_clang`, `test_flags_flang`) to `lit.cfg` so that
compiler-specific flags for Clang and Flang can be specified.

GitOrigin-RevId: 49efb082cced5140fc4ea4476811f697a157bbfc
11 files changed