Maintenance and merging of Valgrind and Valgrind-Freya code paths (#2362)

None of the code paths have been tested for long and especially the
Freya code paths have been long abandoned. This patch:
- Merges Valgrind-Freya into Valgrind code path (there should be no
  need to choose between them, Valgrind should work just fine).
- Removes leftover code (`VALGRIND_FREYA_CHECK_MEMPOOL_REQUEST` and
  `valgrind_freya_mempool_request`).
- Adds `JMEM_` prefix to Valgrind-related macros (to correctly
  leave the `VALGRIND_` prefix to Valgrind).
- Moves the definition of the Valgrind-related macros to a common
  header to avoid duplication.

Note: Adding a CI job to perform Valgrind Memchecks is left for
follow-up as it turns out to be excessively slow (>50 mins for a
`--jerry-tests --jerry-test-suite` run, and even a simple
`--jerry-tests` may get terminated prematurely because of timeout
issues).

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/jcontext/jcontext.h

@@ -131,11 +131,6 @@ typedef struct
 #ifdef JMEM_STATS
   jmem_heap_stats_t jmem_heap_stats; /**< heap's memory usage statistics */
 #endif /* JMEM_STATS */
-
-#ifdef JERRY_VALGRIND_FREYA
-  uint8_t valgrind_freya_mempool_request; /**< Tells whether a pool manager
-                                           *   allocator request is in progress */
-#endif /* JERRY_VALGRIND_FREYA */
 } jerry_context_t;
 
 #ifndef CONFIG_ECMA_LCACHE_DISABLE