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/base/cprintf.cc

@@ -142,7 +142,7 @@ Print::processFlag()
 
           case 'X':
             fmt.uppercase = true;
-            GEM5_FALLTHROUGH;
+            [[fallthrough]];
           case 'x':
             fmt.base = Format::Hex;
             fmt.format = Format::Integer;
@@ -164,7 +164,7 @@ Print::processFlag()
 
           case 'G':
             fmt.uppercase = true;
-            GEM5_FALLTHROUGH;
+            [[fallthrough]];
           case 'g':
             fmt.format = Format::Floating;
             fmt.floatFormat = Format::Best;
@@ -173,7 +173,7 @@ Print::processFlag()
 
           case 'E':
             fmt.uppercase = true;
-            GEM5_FALLTHROUGH;
+            [[fallthrough]];
           case 'e':
             fmt.format = Format::Floating;
             fmt.floatFormat = Format::Scientific;
@@ -220,7 +220,7 @@ Print::processFlag()
                 fmt.fillZero = true;
                 break;
             }
-            GEM5_FALLTHROUGH;
+            [[fallthrough]];
           case '1':
           case '2':
           case '3':