Implement common external function handlers in jerry-ext (#1787)

Added `handler` module to `jerry-ext` to contain implementation of
commonly used external function handlers: `assert`, `gc`, and
`print`.

Also adapted jerry-main to use jerry-ext/handler

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/include/jerryscript-port.h

@@ -62,7 +62,7 @@ typedef enum
  *
  * Example: a libc-based port may implement this with exit() or abort(), or both.
  */
-void jerry_port_fatal (jerry_fatal_code_t code);
+void jerry_port_fatal (jerry_fatal_code_t code) __attribute__((noreturn));
 
 /*
  *  I/O Port API