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/builtin-objects/ecma-builtin-math.c

@@ -122,7 +122,7 @@ ecma_builtin_math_object_max_min (bool is_max, /**< 'max' or 'min' operation */
       ecma_fast_free_value (value);
     }
 
-    if (unlikely (ecma_number_is_nan (arg_num)))
+    if (JERRY_UNLIKELY (ecma_number_is_nan (arg_num)))
     {
       result_num = arg_num;
       break;