[DFSan] Change shadow and origin memory layouts to match MSan.

Previously on x86_64:

  +--------------------+ 0x800000000000 (top of memory)
  | application memory |
  +--------------------+ 0x700000008000 (kAppAddr)
  |                    |
  |       unused       |
  |                    |
  +--------------------+ 0x300000000000 (kUnusedAddr)
  |       origin       |
  +--------------------+ 0x200000008000 (kOriginAddr)
  |       unused       |
  +--------------------+ 0x200000000000
  |   shadow memory    |
  +--------------------+ 0x100000008000 (kShadowAddr)
  |       unused       |
  +--------------------+ 0x000000010000
  | reserved by kernel |
  +--------------------+ 0x000000000000

  MEM_TO_SHADOW(mem) = mem & ~0x600000000000
  SHADOW_TO_ORIGIN(shadow) = kOriginAddr - kShadowAddr + shadow

Now for x86_64:

  +--------------------+ 0x800000000000 (top of memory)
  |    application 3   |
  +--------------------+ 0x700000000000
  |      invalid       |
  +--------------------+ 0x610000000000
  |   ...