doc/contributing/coding_style: change example to use __fallthrough

While the fall through comment is sufficient for gcc to notice that the
fall-through is intentional, clang requires a special attribute which
also works for gcc. Update the documentation to use this attribute
instead of the comment.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I08dbac8ff1f9e04770a03fb74fabf0d397b50989
Reviewed-on: https://review.coreboot.org/c/coreboot/+/71102
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

Documentation/contributing/coding_style.md

@@ -66,7 +66,7 @@ case 'm':
 case 'K':
 case 'k':
 	mem <<= 10;
-	/* fall through */
+	__fallthrough;
 default:
 	break;
 }