powerpc: Fixes to get the merged kernel to boot on powermac.

This merges ppc_ksyms.c, puts back the actual do_execve call in
sys_execve, makes init_MMU call find_end_of_memory rather than
ppc_md.find_end_of_memory (every platform has a device tree
with a /memory node now, right?) and fixes some problems with the
mpic initialization on newworld powermacs.

Signed-off-by: Paul Mackerras <paulus@samba.org>