Apple M1: OS version checking for MAP_JIT

- Added MacOS version checking around MAP_JIT to prepare code for x86 MAP_JIT

Source/Core/Common/MemoryUtil.cpp

@@ -41,7 +41,9 @@ void* AllocateExecutableMemory(size_t size)
 #else
   int map_flags = MAP_ANON | MAP_PRIVATE;
 #if defined(_M_ARM_64) && defined(__APPLE__)
-  map_flags |= MAP_JIT;
+  // This check is in place to prepare for x86_64 MAP_JIT support.
+  if (__builtin_available(macOS 10.14, *))
+    map_flags |= MAP_JIT;
 #endif
   void* ptr = mmap(nullptr, size, PROT_READ | PROT_WRITE | PROT_EXEC, map_flags, -1, 0);
   if (ptr == MAP_FAILED)