Blackfin arch: unify the duplicated portions of __start and split...

Blackfin arch: unify the duplicated portions of __start and split mach-specific pieces into _mach_early_start where they will be easier to trim over time

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>