Add hinting to improve ASLR for macos (all allocations) and linux (newly added allocation types).

src/platform.h

Index: src/platform.h
===================================================================
--- src/platform.h	(revision 9508)
+++ src/platform.h	(working copy)
@@ -178,6 +178,9 @@
   // Assign memory as a guard page so that access will cause an exception.
   static void Guard(void* address, const size_t size);
 
+  // Generate a random address to be used for hinting mmap().
+  static void* GetRandomMmapAddr();
+
   // Get the Alignment guaranteed by Allocate().
   static size_t AllocateAlignment();