[ARM] Mark labels in skipAlignedDPRCS2Spills as fallthrough (NFC).

The comment at the top of the switch statement indicates that the
fall-through behavior is intentional. By using LLVM_FALLTHROUGH,
-Wimplicit-fallthrough are silenced, which is enabled by default in GCC
7.

llvm-svn: 309272

llvm/lib/Target/ARM/ARMFrameLowering.cpp

@@ -1283,9 +1283,11 @@ skipAlignedDPRCS2Spills(MachineBasicBlock::iterator MI,
   case 7:
     ++MI;
     assert(MI->mayStore() && "Expecting spill instruction");
+    LLVM_FALLTHROUGH;
   default:
     ++MI;
     assert(MI->mayStore() && "Expecting spill instruction");
+    LLVM_FALLTHROUGH;
   case 1:
   case 2:
   case 4: