[flang][runtime] Fix empty FINDLOC() results (#75251)

When FINDLOC() can't find its target value among the unmasked array
elements, it must return a zero result. Its implementation doesn't
sufficiently distinguish a zero result from a hit in an array with lower
bound(s) less than one. Fix by adding a flag to distinguish the case
with no hits from cases with hits.

Fixes llvm-test-suite/Fortran/gfortran/regression/findloc_6.f90.