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/jrt/jrt-fatals.c

@@ -26,7 +26,7 @@
  * If !JERRY_NDEBUG and code != 0, print status code with description
  * and call assertion fail handler.
  */
-void __noreturn
+void JERRY_ATTR_NORETURN
 jerry_fatal (jerry_fatal_code_t code) /**< status code */
 {
 #ifndef JERRY_NDEBUG
@@ -67,7 +67,7 @@ jerry_fatal (jerry_fatal_code_t code) /**< status code */
 /**
  * Handle failed assertion
  */
-void __noreturn
+void JERRY_ATTR_NORETURN
 jerry_assert_fail (const char *assertion, /**< assertion condition string */
                    const char *file, /**< file name */
                    const char *function, /**< function name */
@@ -85,7 +85,7 @@ jerry_assert_fail (const char *assertion, /**< assertion condition string */
 /**
  * Handle execution of control path that should be unreachable
  */
-void __noreturn
+void JERRY_ATTR_NORETURN
 jerry_unreachable (const char *file, /**< file name */
                    const char *function, /**< function name */
                    const uint32_t line) /**< line */