simulator: Hopefully fix red introduced in 5b61c79cf
This problem only happened with GCC7 (and older?) simulator builds, and
was introduced by an incorrect test that always stripped our default
optimization (-Os) in simulator builds.
The fix is to correct the test to only strip -Os if the user explicitly
enables a debug build.
Also, use -Og instead of (implicit) -O0, as per GCC recommendations.
Change-Id: I61091b3cdf5323504961c0bb2d80aa65049eb8fb