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/kern/linux/printk.cc

@@ -104,7 +104,7 @@ printk(std::string &str, ThreadContext *tc, Addr format_ptr,
                 break;
               case 'P':
                 format = true;
-                M5_FALLTHROUGH;
+                GEM5_FALLTHROUGH;
               case 'p':
                 hexnum = true;
                 break;