[libunwind] Set up .arcconfig to point to new Diffusion UNW repository

[benhamilton]

Summary

We want to automatically copy the appropriate mailing list
for review requests to the libunwind 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
"UNW" for libunwind:

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

This explicitly updates libunwind's .arcconfig to point to the new
UNW repository in Diffusion, which will let us use a Herald rule
to copy the correct list on reviews.

[benhamilton]

In favor of D41918.

.arcconfig

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