[flang][runtime] Better real MOD/MODULO results (#77167)

The Fortran standard defines real MOD and MODULO with expressions like
MOD(a,p) = a - AINT(a/p)*p. Unfortunately, these definitions have poor
accuracy when a is much larger in magnitude than p, and every Fortran
compiler uses better algorithms instead.

Fixes llvm-test-suite/Fortran/gfortran/regression/mod_large_1.f90.