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/include/jerryscript-port.h

@@ -20,6 +20,8 @@
 #include <stdint.h>
 #include <stdio.h>
 
+#include "jerryscript-compiler.h"
+
 #ifdef __cplusplus
 extern "C"
 {
@@ -62,7 +64,7 @@ typedef enum
  *
  * Example: a libc-based port may implement this with exit() or abort(), or both.
  */
-void jerry_port_fatal (jerry_fatal_code_t code) __attribute__((noreturn));
+void JERRY_ATTR_NORETURN jerry_port_fatal (jerry_fatal_code_t code);
 
 /*
  *  I/O Port API
@@ -96,7 +98,7 @@ typedef enum
  * Example: a libc-based port may implement this with vfprintf(stderr) or
  * vfprintf(logfile), or both, depending on log level.
  */
-void jerry_port_log (jerry_log_level_t level, const char *format, ...) __attribute__ ((format (printf, 2, 3)));
+void JERRY_ATTR_FORMAT (printf, 2, 3) jerry_port_log (jerry_log_level_t level, const char *format, ...);
 
 /*
  * Date Port API