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-exceptions.c

@@ -282,7 +282,7 @@ ecma_raise_standard_error_with_format (ecma_standard_error_t error_type, /**< er
       ecma_string_t *arg_string_p;
       const ecma_value_t arg_val = va_arg (args, ecma_value_t);
 
-      if (unlikely (ecma_is_value_object (arg_val)))
+      if (JERRY_UNLIKELY (ecma_is_value_object (arg_val)))
       {
         ecma_object_t *arg_object_p = ecma_get_object_from_value (arg_val);
         lit_magic_string_id_t class_name = ecma_object_get_class_name (arg_object_p);