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-poolman.c

@@ -76,7 +76,7 @@ jmem_pools_finalize (void)
  * @return pointer to allocated chunk, if allocation was successful,
  *         or NULL - if not enough memory.
  */
-inline void * __attr_hot___ __attr_always_inline___
+inline void * JERRY_ATTR_HOT JERRY_ATTR_ALWAYS_INLINE
 jmem_pools_alloc (size_t size) /**< size of the chunk */
 {
 #ifdef JMEM_GC_BEFORE_EACH_ALLOC
@@ -131,7 +131,7 @@ jmem_pools_alloc (size_t size) /**< size of the chunk */
 /**
  * Free the chunk
  */
-inline void __attr_hot___ __attr_always_inline___
+inline void JERRY_ATTR_HOT JERRY_ATTR_ALWAYS_INLINE
 jmem_pools_free (void *chunk_p, /**< pointer to the chunk */
                  size_t size) /**< size of the chunk */
 {