Interestingly, AuroraFox won't have to do this, and neither will TenFourFox 18, because both of them are built with gcc 4.6 and can use the compiler's intrinsics. This will only be an issue for 17ESR because it is the last branch to be built with 4.0.1. I'm hoping to get this working maybe next week sometime for a test release prior to beginning work on 18, because I'd like to fix it for 17.0.1.
The larger problem of such locks being slow in general, however, means we'll probably have to come up with a better way of handling locking on PPC OS X, or see if some of the areas that use PR_Lock and friends could be better written with atomic intrinsics. Mozilla might even want some of these patches, since it doesn't look like Apple improved this a lot (Grand Central Dispatch is just a lightweight way around the problem).