Reland [OpenMP][Fix] libomptarget Fortran tests (#76189)

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.

This patch re-lands: #74543. The error was caused by having:
```
config.substitutions.append(("%flags", config.test_flags))
config.substitutions.append(("%flags_clang", config.test_flags_clang))
config.substitutions.append(("%flags_flang", config.test_flags_flang))
```
when instead it has to be:
```
config.substitutions.append(("%flags_clang", config.test_flags_clang))
config.substitutions.append(("%flags_flang", config.test_flags_flang))
config.substitutions.append(("%flags", config.test_flags))
```
because LIT replaces with the first longest sub-string match.