[llvm-exegesis] Add myself as an llvm-exegesis maintainer

More people have been interested in exegesis recently, so having a point
of contact would probably be good. Exegesis also never had proper code
owners in the previous system, so adding them now is a decent step
forward.

I'm nominating myself as I'm interested in pushing the project further,
and have a decent amount of experience with the code base. Two of the
original authors, Clement and Guillaume, now have different priorities
at work and are thus not as invested into maintaining exegesis anymore.

Author: boomanaiden154

llvm/Maintainers.md

@@ -407,6 +407,11 @@ [email protected] (email), [MaskRay](https://github.com/MaskRay) (GitHub)
 Teresa Johnson \
 [email protected] (email), [teresajohnson](https://github.com/teresajohnson) (GitHub)
 
+#### llvm-exegesis
+
+Aiden Grossman \
+[email protected] (email), [boomanaiden154](https://github.com/boomanaiden154) (Github)
+
 ### Other
 
 #### Release management