Reland [OpenMP][Fix] libomptarget Fortran tests

[fabianmcg]

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.