Prop hashmap allocation strategy based on gc pattern is implemented. (#1429)

The algorithm is the following:
* introduced a counter variable, which value can be [0-4]
* if its value is 0, property hashmap allocation is enabled
* JMEM_FREE_UNUSED_MEMORY_SEVERITY_LOW -> decrease the counter
* JMEM_FREE_UNUSED_MEMORY_SEVERITY_HIGH -> increase the number
* if JMEM_FREE_UNUSED_MEMORY_SEVERITY_HIGH happens twice in a row increase the counter to 4

According to the measurements this algorithm provides better runtime results in low memory conditions.

JerryScript-DCO-1.0-Signed-off-by: István Kádár ikadar@inf.u-szeged.hu

jerry-core/ecma/base/ecma-globals.h

@@ -1002,6 +1002,22 @@ typedef enum
   ECMA_GC_COLOR__COUNT /**< number of colors */
 } ecma_gc_color_t;
 
+#ifndef CONFIG_ECMA_PROPERTY_HASHMAP_DISABLE
+
+/**
+ * The lowest state of the ecma_prop_hashmap_alloc_state counter.
+ * If ecma_prop_hashmap_alloc_state other other than this value, it is
+ * disabled.
+ */
+#define ECMA_PROP_HASHMAP_ALLOC_ON 0
+
+/**
+ * The highest state of the ecma_prop_hashmap_alloc_state counter.
+ */
+#define ECMA_PROP_HASHMAP_ALLOC_MAX 4
+
+#endif /* !CONFIG_ECMA_PROPERTY_HASHMAP_DISABLE */
+
 /**
  * Number of values in a literal storage item
  */