Cygwin: Add 'fallthrough' pseudo keyword for switch/case use

This patch has been inspired by the Linux kernel patch

  294f69e662d1 compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

written by Joe Perches <joe AT perches DOT com> based on an idea from
Dan Carpenter <dan DOT carpenter AT oracle DOT com>.  The following text
is from the original log message:

Reserve the pseudo keyword 'fallthrough' for the ability to convert the
various case block /* fallthrough */ style comments to appear to be an
actual reserved word with the same gcc case block missing fallthrough
warning capability.

All switch/case blocks now should end in one of:

	break;
	fallthrough;
	goto <label>;
	return [expression];
	continue;

In C mode, GCC supports the __fallthrough__ attribute since 7.1,
the same time the warning and the comment parsing were introduced.

Cygwin-only: add an explicit -Wimplicit-fallthrough=5 to the build
flags.

winsup/cygwin/libc/base64.c

@@ -282,7 +282,7 @@ b64_pton(char const *src, unsigned char *target, size_t targsize)
 				return (-1);
 			ch = *src++;		/* Skip the = */
 			/* Fall through to "single trailing =" case. */
-			/* FALLTHROUGH */
+			fallthrough;
 
 		case 3:		/* Valid, means two bytes of info */
 			/*