[flang] Fix complex libm use logic problems

Fix the various complex libm selection logic issues from D155310:
- disableMlirComplex is set to false. This means that using mlir complex
is enabled. Yet, the current code still selects libm.
- If we enable mlir complex, we should not check if use approx is
enabled. Namely, we should use mlir complex either if we enable mlir
complex OR use approx is enabled.

To fix the issues, we flip the logic of `disableMlirComplex` to enable
instead. We set it to false by default since the intention from D155310
is to use libm by default. Then we use a logical `&&` with use approx
so that we select libm when BOTH mlir complex and use approx are
disabled.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D155737