ppc: reduce size of Forth dictionary to 384K

gcc 9 emits a fatal error when building OpenBIOS for ppc because the size of the
generated image is greater than the available ROM area when building with -O0.

Reduce the size of the Forth dictionary from 512K to 384K to allow sufficient
space for generating OpenBIOS images suitable for debugging.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>