[LLVM] Update backend maintainer (llvm#116622)

We currently list Evan Cheng as the fallback maintainer for the LLVM
backend. However, their last contribution dates back to 2014.

I'd like to nominate arsenm instead, who is our most active backend
reviewer.

llvm/Maintainers.md

@@ -121,8 +121,8 @@ [email protected] (email), [davidtrevelyan](https://github.com/davidtrev
 
 #### Parts of code generator not covered by someone else
 
-Evan Cheng \
-evan.cheng@apple.com (email)
+Matt Arsenault \
+Matthew.Arsenault@amd.com, [email protected] (email), [arsenm](https://github.com/arsenm) (GitHub)
 
 #### SelectionDAG
 
@@ -469,6 +469,7 @@ [email protected] (email), [lattner](https://github.com/lattner) (GitHub), clattn
 ### Inactive or former component maintainers
 
 Justin Bogner ([email protected], [bogner](https://github.com/bogner)) -- SelectionDAG \
+Evan Cheng ([email protected]) -- Parts of code generator not covered by someone else \
 Hans Wennborg ([email protected], [zmodem](https://github.com/zmodem)) -- Release management \
 
 ### Former maintainers of removed components