When using virtual functions while RTTI is enabled, the compiler
generates code to support the use of dynamic_cast etc. This code
requires the use of the free store (heap) which is not yet available.
Disabling RTTI also disables the generation of the associated support
code, thus making it possible for us to use virtual functions.

Even though it makes no difference, since the CRT code we pull in from
the toolchain will never touch the red zone in the first place, it is a
good idea to try to be consistent across the build.