[POWERPC] Fix bootwrapper builds with older gcc versions

GCC versions before 3.4 did not support the -mcpu=440 option.  Use
-mcpu=405 for the 4xx specific bootwrapper files, as that has been
around for much longer.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>