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/parser/js/js-parser.c

@@ -1228,7 +1228,7 @@ parse_print_final_cbc (ecma_compiled_code_t *compiled_code_p, /**< compiled code
   }
 
   byte_code_start_p += (unsigned int) (literal_end - register_end) * sizeof (ecma_value_t);
-  if (unlikely (compiled_code_p->status_flags & CBC_CODE_FLAGS_NON_STRICT_ARGUMENTS_NEEDED))
+  if (JERRY_UNLIKELY (compiled_code_p->status_flags & CBC_CODE_FLAGS_NON_STRICT_ARGUMENTS_NEEDED))
   {
     byte_code_start_p += argument_end * sizeof (ecma_value_t);
   }