[compiler-rt] Set up .arcconfig to point to new Diffusion CRT repository

[benhamilton]

Summary

We want to automatically copy the appropriate mailing list
as well as #sanitizers for review requests to the compiler-rt repository.

For context, see the proposal and discussion here:

http://lists.llvm.org/pipermail/cfe-dev/2017-November/056032.html

Similar to D40179, I set up a new Diffusion repository with callsign
"CRT" for compiler-rt:

https://reviews.llvm.org/source/compiler-rt/

This explicitly updates compiler-rt's .arcconfig to point to the new
CRT repository in Diffusion, which will let us use Herald rules H270
and H271.

.arcconfig

 {
-  "project_id" : "compiler-rt",
+  "repository.callsign" : "CRT",
   "conduit_uri" : "https://reviews.llvm.org/"
 }