[POWERPC] Fix hang in start_ldr if _end or _edata is unaligned

Quick fix for lack of memset(__bss_start, 0, _end-__bss_start) in
load_kernel().  If edata is unaligned, the loop will overwrite all
memory because r3 and r4 will never be equal.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>