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/vm/vm-stack.c

@@ -75,7 +75,7 @@ vm_stack_context_abort (vm_frame_ctx_t *frame_ctx_p, /**< frame context */
       {
         ecma_value_t value = chunk_p->items[index];
 
-        if (unlikely (ecma_is_value_collection_chunk (value)))
+        if (JERRY_UNLIKELY (ecma_is_value_collection_chunk (value)))
         {
           ecma_collection_chunk_t *next_chunk_p = ecma_get_collection_chunk_from_value (value);
           jmem_heap_free_block (chunk_p, sizeof (ecma_collection_chunk_t));