Update backend fallback maintainer

Author: nikic

llvm/Maintainers.md

@@ -116,8 +116,8 @@ [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
 
@@ -461,6 +461,7 @@ [email protected] (email), [lattner](https://github.com/lattner) (GitHub), clattn
 
 ### Inactive or former component maintainers
 
+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