AMDGPU: Add missing GK_VOLCANIC_ISLANDS to switch statement

This fixes buildbot failures introduced by r240038.

llvm-svn: 240059

clang/lib/Basic/Targets.cpp

@@ -1861,6 +1861,7 @@ public:
       break;
     case GK_SOUTHERN_ISLANDS:
     case GK_SEA_ISLANDS:
+    case GK_VOLCANIC_ISLANDS:
       DescriptionString = DescriptionStringSI;
       hasFP64 = true;
       hasFMAF = true;