PPC: Unify dcbzl code path

The bit that makes a dcbz instruction a dcbzl instruction was declared as
reserved in ppc32 ISAs. However, hardware simply ignores the bit, making
code valid if it simply invokes dcbzl instead of dcbz even on 750 and G4.

Thus, mark the bit as unreserved so that we properly emulate a simple dcbz
in case we're running on non-G5s.

While at it, also refactor the code to check the 970 special case during
runtime. This way we don't need to differenciate between a 970 dcbz and
any other dcbz anymore. We also allow for future improvements to add e500mc
dcbz handling.

Reported-by: Amadeusz Sławiński <amade@asmblr.net>
Signed-off-by: Alexander Graf <agraf@suse.de>