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.h

@@ -38,10 +38,10 @@ ecma_object_t *
 ecma_arraybuffer_new_object_external (ecma_length_t length,
                                       void *buffer_p,
                                       ecma_object_native_free_callback_t free_cb);
-lit_utf8_byte_t *
-ecma_arraybuffer_get_buffer (ecma_object_t *obj_p) __attr_pure___;
-ecma_length_t
-ecma_arraybuffer_get_length (ecma_object_t *obj_p) __attr_pure___;
+lit_utf8_byte_t * JERRY_ATTR_PURE
+ecma_arraybuffer_get_buffer (ecma_object_t *obj_p);
+ecma_length_t JERRY_ATTR_PURE
+ecma_arraybuffer_get_length (ecma_object_t *obj_p);
 bool
 ecma_is_arraybuffer (ecma_value_t val);