After many cycles of compiling and testing ppc64 kernels, I finally found out what causes 2.6.12-rc-mm kernels to lock up my PowerMac G5 regularly: It’s the recent changes in kernel/timer.c.
When I exclude the timer-* patches from the mm series everything works fine again.

I have not found a bug in these patches yet but I am pretty sure that it is no GCC bug.