objtool: Use 'the fallthrough' pseudo-keyword

Replace the existing /* fallthrough */ comments with the
new 'fallthrough' pseudo-keyword macro:

  https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org

tools/objtool/arch/x86/decode.c

@@ -291,7 +291,7 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 		switch (modrm_reg & 7) {
 		case 5:
 			imm = -imm;
-			/* fallthrough */
+			fallthrough;
 		case 0:
 			/* add/sub imm, %rsp */
 			ADD_OP(op) {
@@ -375,7 +375,7 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 			break;
 		}
 
-		/* fallthrough */
+		fallthrough;
 	case 0x88:
 		if (!rex_w)
 			break;
@@ -656,7 +656,7 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 			break;
 		}
 
-		/* fallthrough */
+		fallthrough;
 
 	case 0xca: /* retf */
 	case 0xcb: /* retf */