[flang] Use libm functions for complex operations by default

This patch changes the default lowering for complex operations to use
the more accurate libm operations as opposed to the mlir complex
operations. This is necessary due to precision issues in the mlir
complex dialect that cause failures in e.g. the LAPACK tests.

The mlir complex dialect lowering will still be used when
`-fapprox-func` is set (and by extension `-ffast-math` and `-Ofast`)

Differential Revision: https://reviews.llvm.org/D155310