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

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.

Reviewers: krytarowski, joerg, dvyukov, vitalybuka, sammccall

Reviewed By: sammccall

Subscribers: dlj, bkramer, dberris, llvm-commits, sammccall, klimek, #sanitizers

Differential Revision: https://reviews.llvm.org/D40498

llvm-svn: 319954