Silicon/Styx: get rid of NUM_CORES preprocessor define on command line

Instead of relying on the compiler command line to pass the value
of NUM_CORES as a preprocessor define, use the value of the PcdCoreCount
PCD that we already set in the platform .DSC.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>