hello-wuffs-c: update endwhile syntax

Also fix lang/check

hello-wuffs-c/parse.wuffs

@@ -37,7 +37,7 @@ pub func parser.parse?(src: base.io_reader) {
         // Uncomment this assertion to see what facts are known here.
         // assert false
         this.val = (10 * this.val) + (c as base.u32)
-    } endwhile
+    }
 }
 
 pub func parser.value() base.u32 {

lang/check/check_test.go

@@ -86,7 +86,7 @@ func TestCheck(tt *testing.T) {
 			{
 				// Redundant, but shows the labeled jump syntax.
 				continue.label
-			} endwhile.label
+			}.label
 		}
 	`) + "\n"