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/sec_posixacl.cc

@@ -453,7 +453,7 @@ acl_set_tag_type (acl_entry_t entry_d, acl_tag_t tag_type)
 	  case ACL_MASK:
 	  case ACL_OTHER:
 	    acl->entry[idx].a_id = ACL_UNDEFINED_ID;
-	    /*FALLTHRU*/
+	    fallthrough;
 	  case ACL_USER:
 	  case ACL_GROUP:
 	    acl->entry[idx].a_type = tag_type;