Port pdumper mmap to AIX

* src/pdumper.c (needs_mmap_retry_p) [_AIX]: Return true.
Problem observed on AIX 7.1 and 7.2 in GCC compile farm.

src/pdumper.c

@@ -4734,7 +4734,7 @@ dump_mmap_release_vm (struct dump_memory_map *map)
 static bool
 needs_mmap_retry_p (void)
 {
-#if defined (CYGWIN) || VM_SUPPORTED == VM_MS_WINDOWS
+#if defined CYGWIN || VM_SUPPORTED == VM_MS_WINDOWS || defined _AIX
   return true;
 #else
   return false;