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/ecma/operations/ecma-promise-object.c

@@ -41,7 +41,7 @@
  * @return true - if the object is a promise.
  *         false - otherwise.
  */
-inline bool __attr_always_inline___
+inline bool JERRY_ATTR_ALWAYS_INLINE
 ecma_is_promise (ecma_object_t *obj_p) /**< points to object */
 {
   return ecma_object_class_is (obj_p, LIT_MAGIC_STRING_PROMISE_UL);
@@ -66,7 +66,7 @@ ecma_promise_get_result (ecma_object_t *obj_p) /**< points to promise object */
 /**
  * Set the PromiseResult of promise.
  */
-inline void __attr_always_inline___
+inline void JERRY_ATTR_ALWAYS_INLINE
 ecma_promise_set_result (ecma_object_t *obj_p, /**< points to promise object */
                          ecma_value_t result) /**< the result value */
 {
@@ -84,7 +84,7 @@ ecma_promise_set_result (ecma_object_t *obj_p, /**< points to promise object */
  *
  * @return the state's enum value
  */
-inline uint8_t __attr_always_inline___
+inline uint8_t JERRY_ATTR_ALWAYS_INLINE
 ecma_promise_get_state (ecma_object_t *obj_p) /**< points to promise object */
 {
   JERRY_ASSERT (ecma_is_promise (obj_p));
@@ -95,7 +95,7 @@ ecma_promise_get_state (ecma_object_t *obj_p) /**< points to promise object */
 /**
  * Set the PromiseState of promise.
  */
-inline void __attr_always_inline___
+inline void JERRY_ATTR_ALWAYS_INLINE
 ecma_promise_set_state (ecma_object_t *obj_p, /**< points to promise object */
                         uint8_t state) /**< the state */
 {