[libcxxabi] Set up .arcconfig to point to new Diffusion CXXA repository

[benhamilton]

Summary

We want to automatically copy the appropriate mailing list
for review requests to the libc++abi repository.

For context, see the proposal and discussion here:

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

Similar to D40500, I set up a new Diffusion repository with callsign
"CXXA" for libc++abi:

https://reviews.llvm.org/source/libcxxabi/

This explicitly updates libcxxabi's .arcconfig to point to the new
CXX repository in Diffusion, which will let us use Herald rule H268.

[phosek]

LGTM

.arcconfig

 {
-  "project_id" : "libcxxabi",
+  "repository.callsign" : "CXXA",
   "conduit_uri" : "https://reviews.llvm.org/"
 }