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-objects-general.c

@@ -431,10 +431,10 @@ ecma_op_general_object_define_own_property (ecma_object_t *object_p, /**< the ob
   {
     /* No action required. */
   }
-  else if (likely (property_desc_type == current_property_type))
+  else if (JERRY_LIKELY (property_desc_type == current_property_type))
   {
     /* If property is configurable, there is no need for checks. */
-    if (unlikely (!is_current_configurable))
+    if (JERRY_UNLIKELY (!is_current_configurable))
     {
       if (property_desc_type == ECMA_PROPERTY_TYPE_NAMEDDATA)
       {