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/strptime.cc

@@ -524,7 +524,7 @@ literal:
 
 		case 'k':	/* The hour (24-hour clock representation). */
 			LEGAL_ALT(0);
-			/* FALLTHROUGH */
+			fallthrough;
 		case 'H':
 			LEGAL_ALT(ALT_O);
 			bp = conv_num(bp, &tm->tm_hour, 0, 23, ALT_DIGITS);
@@ -532,7 +532,7 @@ literal:
 
 		case 'l':	/* The hour (12-hour clock representation). */
 			LEGAL_ALT(0);
-			/* FALLTHROUGH */
+			fallthrough;
 		case 'I':
 			LEGAL_ALT(ALT_O);
 			bp = conv_num(bp, &tm->tm_hour, 1, 12, ALT_DIGITS);