[POWERPC] Fix zImage.coff generation for 32-bit pmac

Commit 9da82a6d

 inadvertently
removed the platform override for zImage.coff to be generated
with pmaccoff.   Rather than add a special makefile rule,
change the platform for which the wrapper platform uses
the special rules.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>