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-arraybuffer-object.c

@@ -171,7 +171,7 @@ ecma_is_arraybuffer (ecma_value_t target) /**< the target value */
  *
  * @return ecma_length_t, the length of the arraybuffer
  */
-ecma_length_t __attr_pure___
+ecma_length_t JERRY_ATTR_PURE
 ecma_arraybuffer_get_length (ecma_object_t *object_p) /**< pointer to the ArrayBuffer object */
 {
   JERRY_ASSERT (ecma_object_class_is (object_p, LIT_MAGIC_STRING_ARRAY_BUFFER_UL));
@@ -185,7 +185,7 @@ ecma_arraybuffer_get_length (ecma_object_t *object_p) /**< pointer to the ArrayB
  *
  * @return pointer to the data buffer
  */
-inline lit_utf8_byte_t * __attr_pure___ __attr_always_inline___
+inline lit_utf8_byte_t * JERRY_ATTR_PURE JERRY_ATTR_ALWAYS_INLINE
 ecma_arraybuffer_get_buffer (ecma_object_t *object_p) /**< pointer to the ArrayBuffer object */
 {
   JERRY_ASSERT (ecma_object_class_is (object_p, LIT_MAGIC_STRING_ARRAY_BUFFER_UL));