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-conversion.c

@@ -372,7 +372,7 @@ ecma_op_to_string (ecma_value_t value) /**< ecma value */
 {
   ecma_check_value_type_is_spec_defined (value);
 
-  if (unlikely (ecma_is_value_object (value)))
+  if (JERRY_UNLIKELY (ecma_is_value_object (value)))
   {
     ecma_value_t ret_value = ECMA_VALUE_EMPTY;