net/samba410: disable DEBUG on powerpc64

-O0 breaks assumption that the compiler is optimizing functions, which leads to unresolved symbols:
ld: error: /wrkdirs/usr/ports/net/samba410/work/samba-4.10.13/bin/default/source3/libsmbd-base-samba4.so: undefined reference to unsafe_string_function_usage_here_NTSTATUS

DEBUG option probably shouldn't be on by default anyway, but I assume there must be some reason for it.