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/arch/arm/tlb.cc

@@ -978,7 +978,7 @@ TLB::checkPermissions64(TlbEntry *te, const RequestPtr &req, Mode mode,
                 grant_read = false;
                 break;
             }
-            M5_FALLTHROUGH;
+            GEM5_FALLTHROUGH;
           case EL3:
             {
                 uint8_t perm = (ap & 0x2) | xn;