Add target maintainer information for powerpc64-unknown-linux-musl
We intend to fix the outstanding issues on the target and eventually promote it to tier 2. We have the capacity to maintain this target in the future and already perform regular builds of rustc for this target.
Currently, all host tools except miri build fine, but I have a patch for libffi-sys to make miri also compile fine for this target that is [pending review](https://github.com/tov/libffi-rs/pull/100).
While at it, add an option for the musl root for this target.
I also added a kernel version requirement, which is rather arbitrarily chosen, but it matches our tier 2 powerpc64le-unknown-linux-musl target so I think it is a good fit.
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