Add a 64-bit ARM builder to the CQ.

It's possible this rarely breaks, but I do write ARMv8 code frequently
enough that it'll help prevent broken trees.

BUG=skia:

Review URL: https://codereview.chromium.org/1209763003
diff --git a/infra/branch-config/cq.cfg b/infra/branch-config/cq.cfg
index e2f3e41..f95292d 100644
--- a/infra/branch-config/cq.cfg
+++ b/infra/branch-config/cq.cfg
@@ -34,6 +34,7 @@
       name: "client.skia.compile"
       builders { name: "Build-Mac10.8-Clang-x86_64-Release-Trybot" }
       builders { name: "Build-Ubuntu-Clang-x86_64-Debug-Trybot" }
+      builders { name: "Build-Ubuntu-GCC-Arm64-Debug-Android-Trybot" }
       builders { name: "Build-Ubuntu-GCC-Arm7-Debug-Android-Trybot" }
       builders { name: "Build-Ubuntu-GCC-Mips-Debug-Android-Trybot" }
       builders { name: "Build-Ubuntu-GCC-x86_64-Release-Trybot" }