Optimized glibc builds can influence system performance greatly. This is especially true on powerpc systems, where a lot of cpu variants exist. Up to now, the glibc package provides an additional library set for power6, while standard libraries are compiled for 604 or common? I'm just guessing... While it would be a waste of space to support all varities of cpus in a single glibc package, I suggest to split it into several sub-packages, e.g. - glibc-common (required) - glibc-power6 (with power6 directory, compiled with -mcpu=power6) - glibc-cell (with ppc-cell-be directory, compiled with -mcpu=970 or -mcpu=cell) - ... The same holds also for kernels. I found noticeable performace improvements using my own kernel and glibc (build with -mcpu=970 or mcpu=cell using ppu-gcc) on my PS3 system (but not on my G3/750).
This would be a maintainance nightmare, plus glibc on ppc and ppc64 already builds about 3 times longer or more than on other architectures.