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-helpers-json.c

@@ -115,7 +115,7 @@ ecma_builtin_helper_json_create_separated_properties (ecma_collection_header_t *
     ecma_string_t *current_p = ecma_get_string_from_value (*ecma_value_p);
     ecma_value_p = ecma_collection_iterator_next (ecma_value_p);
 
-    if (likely (!first))
+    if (JERRY_LIKELY (!first))
     {
       properties_str_p = ecma_concat_ecma_strings (properties_str_p, separator_p);
     }