Just leave some members in even if they are unused on e.g.
32-bit PPC or user-mode emulation.  This avoids complications
when using PowerPCCPUClass in code that is compiled just
once (because it applies to both 32-bit and 64-bit PPC
for example) but still needs to peek at PPC-specific members.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>