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/jcontext/jcontext.h

@@ -187,7 +187,7 @@ struct jerry_instance_t
 
 #ifndef JERRY_SYSTEM_ALLOCATOR
 
-static inline jmem_heap_t * __attr_always_inline___
+static inline jmem_heap_t * JERRY_ATTR_ALWAYS_INLINE
 jerry_context_get_current_heap (void)
 {
   return JERRY_GET_CURRENT_INSTANCE ()->heap_p;
@@ -207,7 +207,7 @@ jerry_context_get_current_heap (void)
 
 #ifndef CONFIG_ECMA_LCACHE_DISABLE
 
-static inline jerry_hash_table_t * __attr_always_inline___
+static inline jerry_hash_table_t * JERRY_ATTR_ALWAYS_INLINE
 jerry_context_get_current_lcache (void)
 {
   return (jerry_hash_table_t *) (JERRY_GET_CURRENT_INSTANCE ()->lcache_p);