misc: Replace GEM5_FALLTHROUGH with [[fallthrough]].

Now that the [[fallthrough]] attribute is standard (as of c++-17), we
can use it directly instead of hiding it behind a macro.

Change-Id: I4d11e35b619532b1a3fd8d042265e18c80d86f9b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48505
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/arm/smmu_v3.cc

@@ -253,7 +253,7 @@ SMMUv3::runProcessAtomic(SMMUProcess *proc, PacketPtr pkt)
                     pkt = action.pkt;
                     break;
                 }
-                GEM5_FALLTHROUGH;
+                [[fallthrough]];
             case ACTION_SEND_REQ_FINAL:
                 delay += requestPort.sendAtomic(action.pkt);
                 pkt = action.pkt;
@@ -309,7 +309,7 @@ SMMUv3::runProcessTiming(SMMUProcess *proc, PacketPtr pkt)
 
                 break;
             }
-            GEM5_FALLTHROUGH;
+            [[fallthrough]];
         case ACTION_SEND_REQ_FINAL:
             action.pkt->pushSenderState(proc);
 

src/dev/storage/ide_disk.cc

@@ -687,7 +687,7 @@ IdeDisk::startCommand()
         // Supported DMA commands
       case WDCC_WRITEDMA:
         dmaRead = true;  // a write to the disk is a DMA read from memory
-        GEM5_FALLTHROUGH;
+        [[fallthrough]];
       case WDCC_READDMA:
         if (!(cmdReg.drive & DRIVE_LBA_BIT))
             panic("Attempt to perform CHS access, only supports LBA\n");