powerpc: Require gcc 4.0 on 64-bit

Older versions of gcc had issues with using -maltivec together with
-mcpu of a non altivec capable CPU. We work around it by specifying
-mcpu=970, but the logic is complicated.

In preparation for adding more -mcpu targets, remove the workaround
and just require gcc 4.0 for 64-bit builds.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>