It turns out that I was looking at results from an old tree. In
3.4.x, if you configure --with-cpu=603 and then use -maltivec, the
Altivec extensions are still disabled. That's been changed in 4.0.
We wouldn't want to backport that change to 3.4, as it might result in
silent code changes to user applications, but seeing the test failures
isn't helpful. So, for 3.4 only, I checked in this change, which
ensures that we're building the AltiVec tests for an appropriate CPU.
Thanks to David and Aldy for helping me with this issue.
Tested with a PowerPC GNU/Linux compiler configured --with-cpu=603,
committed on the 3.4 branch.