fix: unify fallthrough annotations

Compiling with -Wimplicit-fallthrough=3 (enabled by -Wextra) produces
many warnings about fallthrough comments either missing or being wrong.
Unify the comments so we comply with -Wextra.

Note that Coverity recommends against using the __attribute__ directive.
Also, zlib does not build with a higher value of -Wimplicit-fallthrough.
Finally, compilers strip comments before expanding macros. As such,
checkpatch's fallthrough annotation (or higher levels of the flag) isn't
really possible.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I060cf4f8dc04c02cbb45cf4ceb69569a8369ccee

lib/libc/snprintf.c

@@ -209,6 +209,7 @@ loop:
 				break;
 			case 'X':
 				capitalise = true;
+				/* fallthrough */
 			case 'x':
 				unum = get_unum_va_args(args, l_count);
 				unsigned_num_print(&s, n, &chars_printed,