arch: FDPIC is not supported on m68k

The current binfmt selection in arch/Config.in allows to select FDPIC
on m68k, which is incorrect. This commit fixes that, and makes sure
FDPIC is the default on Blackfin, while FLAT is the default on m68k.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>