[llvm] [LLVM] Update Sparc maintainer (PR #120430)

[Rainer Orth via llvm-commits]

rorth wrote:

Given @koachan's amazing SPARC work, I'm all for this change.

As for my involvement, I do run the Solaris/sparcv9 buildbot, but my work mostly involves Solaris stuff, not the backend.

https://github.com/llvm/llvm-project/pull/120430