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/jmem/jmem-allocator.c

@@ -56,7 +56,7 @@ jmem_finalize (void)
  *
  * @return packed pointer
  */
-inline jmem_cpointer_t __attr_pure___ __attr_always_inline___
+inline jmem_cpointer_t JERRY_ATTR_PURE JERRY_ATTR_ALWAYS_INLINE
 jmem_compress_pointer (const void *pointer_p) /**< pointer to compress */
 {
   JERRY_ASSERT (pointer_p != NULL);
@@ -90,7 +90,7 @@ jmem_compress_pointer (const void *pointer_p) /**< pointer to compress */
  *
  * @return unpacked pointer
  */
-inline void * __attr_pure___ __attr_always_inline___
+inline void * JERRY_ATTR_PURE JERRY_ATTR_ALWAYS_INLINE
 jmem_decompress_pointer (uintptr_t compressed_pointer) /**< pointer to decompress */
 {
   JERRY_ASSERT (compressed_pointer != JMEM_CP_NULL);