target-ppc: Drop nested TARGET_PPC64 guard for POWER7

It is within a large TARGET_PPC64 section from 970 to 620,
so an #endif /* TARGET_PPC64 */ is confusing. Clean this up.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
