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.h

@@ -164,8 +164,8 @@ void jmem_stats_free_property_bytes (size_t property_size);
 void jmem_heap_get_stats (jmem_heap_stats_t *);
 #endif /* JMEM_STATS */
 
-jmem_cpointer_t jmem_compress_pointer (const void *pointer_p) __attr_pure___;
-void *jmem_decompress_pointer (uintptr_t compressed_pointer) __attr_pure___;
+jmem_cpointer_t JERRY_ATTR_PURE jmem_compress_pointer (const void *pointer_p);
+void * JERRY_ATTR_PURE jmem_decompress_pointer (uintptr_t compressed_pointer);
 
 /**
  * A free memory callback routine type.
@@ -217,7 +217,7 @@ void jmem_run_free_unused_memory_callbacks (jmem_free_unused_memory_severity_t s
  * Get value of pointer from specified compressed pointer value
  */
 #define JMEM_CP_GET_POINTER(type, cp_value) \
-  (((unlikely ((cp_value) == JMEM_CP_NULL)) ? NULL : JMEM_CP_GET_NON_NULL_POINTER (type, cp_value)))
+  (((JERRY_UNLIKELY ((cp_value) == JMEM_CP_NULL)) ? NULL : JMEM_CP_GET_NON_NULL_POINTER (type, cp_value)))
 
 /**
  * Set value of non-null compressed pointer so that it will correspond
@@ -235,7 +235,7 @@ void jmem_run_free_unused_memory_callbacks (jmem_free_unused_memory_severity_t s
   { \
     void *ptr_value = (void *) non_compressed_pointer; \
     \
-    if (unlikely ((ptr_value) == NULL)) \
+    if (JERRY_UNLIKELY ((ptr_value) == NULL)) \
     { \
       (cp_value) = JMEM_CP_NULL; \
     } \