Revert "[flang] IEEE_ARITHMETIC must imply USE IEEE_EXCEPTIONS"

This reverts commit b6a76004. It caused
the following build failure:
```
ninja: error: dependency cycle: include/flang/__fortran_ieee_exceptions.mod -> include/flang/__fortran_ieee_exceptions.mod
```

See e.g.:
* https://lab.llvm.org/buildbot/#/builders/172/builds/9595

To reproduce:
```
cmake -G Ninja \
  -DLLVM_TARGETS_TO_BUILD=host \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_PROJECTS="clang;flang" \
  ../../llvm
ninja check-flang
```