Add minimal platform support documentation for powerpc-unknown-linux-gnuspe
Per https://github.com/rust-lang/rust/pull/137860#issuecomment-2692358259, add minimal platform support documentation, including a brief summary and links to more detailed information about this target.
The added documentation is minimal. This is somewhat similar to [powerpc-unknown-openbsd, which also has no target maintainer](https://doc.rust-lang.org/nightly/rustc/platform-support/powerpc-unknown-openbsd.html). The rest of the template is left to target maintainers.
I also updated powerpc-unknown-linux-muslspe platform support documentation and added link to powerpc-unknown-linux-gnuspe platform support documentation.
cc ```@glaubitz```
cc ```@BKPepe```
r? workingjubilee
```@rustbot``` label +O-PowerPC
38b48203af