add ex087 async 4

exercises/087_async4.zig

@@ -0,0 +1,30 @@
+//
+// It has probably not escaped your attention that we are no
+// longer capturing a return value from foo() because the 'async'
+// keyword returns the frame instead.
+//
+// One way to solve this is to use a global variable.
+//
+// See if you can make this program print "1 2 3 4 5".
+//
+const print = @import("std").debug.print;
+
+var global_counter: i32 = 0;
+
+pub fn main() void {
+    var foo_frame = async foo();
+
+    while (global_counter <= 5) {
+        print("{} ", .{global_counter});
+        ???
+    }
+
+    print("\n", .{});
+}
+
+fn foo() void {
+    while (true) {
+        ???
+        ???
+    }
+}