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/ecma/base/ecma-lcache.c

@@ -50,7 +50,7 @@ ecma_lcache_init (void)
 /**
  * Invalidate specified LCache entry
  */
-static inline void __attr_always_inline___
+static inline void JERRY_ATTR_ALWAYS_INLINE
 ecma_lcache_invalidate_entry (ecma_lcache_hash_entry_t *entry_p) /**< entry to invalidate */
 {
   JERRY_ASSERT (entry_p != NULL);
@@ -66,7 +66,7 @@ ecma_lcache_invalidate_entry (ecma_lcache_hash_entry_t *entry_p) /**< entry to i
  *
  * @return row index
  */
-static inline size_t __attr_always_inline___
+static inline size_t JERRY_ATTR_ALWAYS_INLINE
 ecma_lcache_row_index (jmem_cpointer_t object_cp, /**< compressed pointer to object */
                        lit_string_hash_t name_hash) /**< property name hash */
 {
@@ -144,7 +144,7 @@ ecma_lcache_insert (ecma_object_t *object_p, /**< object */
  * @return a pointer to an ecma_property_t if the lookup is successful
  *         NULL otherwise
  */
-inline ecma_property_t * __attr_always_inline___
+inline ecma_property_t * JERRY_ATTR_ALWAYS_INLINE
 ecma_lcache_lookup (ecma_object_t *object_p, /**< object */
                     const ecma_string_t *prop_name_p) /**< property's name */
 {