ART: Add missing PrettyCause

Add missing name. Refactor so that build breaks next time anyone
forgets adding this.

Bug: 31385027
Test: m test-art-host
Change-Id: I7f5a510b56829aa39e8ca6bcff11c3e9eee95000

runtime/gc/gc_cause.cc

diff --git a/runtime/gc/gc_cause.cc b/runtime/gc/gc_cause.cc
index 1d377a4..7ff845d 100644
--- a/runtime/gc/gc_cause.cc
+++ b/runtime/gc/gc_cause.cc
@@ -38,10 +38,10 @@
     case kGcCauseDebugger: return "Debugger";
     case kGcCauseClassLinker: return "ClassLinker";
     case kGcCauseJitCodeCache: return "JitCodeCache";
-    default:
-      LOG(FATAL) << "Unreachable";
-      UNREACHABLE();
+    case kGcCauseAddRemoveSystemWeakHolder: return "SystemWeakHolder";
   }
+  LOG(FATAL) << "Unreachable";
+  UNREACHABLE();
 }
 
 std::ostream& operator<<(std::ostream& os, const GcCause& gc_cause) {