fix up the compiler flags for good

Commit 399cf571 has been completely fucked up, additionally to both my previous fixes (ifneq->ifeq, 0->O) the GLOBAL_CFLAGS and RELEASE_CFLAGS have been mixed up.
Due to the previous errors this one has been covered up, but now it leads to build errors for non-O3 builds. I didn't notice that until now as I've been testing the fixed O3 flag only.
Note that before my commits the compiler always used O3, regardless  of TARGET_USE_O3.

I've reordered and fixed the commit now, this should finally match the intention of @aaronpoweruser.