Add jerryscript-compiler.h public header to cover compiler incompatibilities (#2313)

In general, public headers should not have compiler-specific
constructs but both the core and the port headers have attributes,
which are non-standard. It's better to factor out such constructs
to a common place (a new header) and hide them behind macros, which
can then be defined on a per-compiler basis.

This patch moves the existing definitions of function attributes and
likely/unlikely builtins to the new header. At the same time, it
unifies the names of these attribute defines and where they are
used. Moreover, it touches on jerry-main and removes the uses of
`__attribute__((unused))` entirely and replaces them with the
elsewhere used `(void) ...` pattern.

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

jerry-core/debugger/debugger.c

@@ -258,7 +258,7 @@ jerry_debugger_sleep (void)
  * @return true - if message is processed successfully
  *         false - otherwise
  */
-inline bool __attr_always_inline___
+inline bool JERRY_ATTR_ALWAYS_INLINE
 jerry_debugger_process_message (uint8_t *recv_buffer_p, /**< pointer to the received data */
                                 uint32_t message_size, /**< message size */
                                 bool *resume_exec_p, /**< pointer to the resume exec flag */