misc: Replace M5_FALLTHROUGH with GEM5_FALLTHROUGH.

Change-Id: I058f311b6d9c284f745bcc915db72236d05db21b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45233
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/storage/ide_disk.cc

@@ -682,7 +682,7 @@ IdeDisk::startCommand()
         // Supported DMA commands
       case WDCC_WRITEDMA:
         dmaRead = true;  // a write to the disk is a DMA read from memory
-        M5_FALLTHROUGH;
+        GEM5_FALLTHROUGH;
       case WDCC_READDMA:
         if (!(cmdReg.drive & DRIVE_LBA_BIT))
             panic("Attempt to perform CHS access, only supports LBA\n");