Attempt to read load_base from memory when needed.

When creating the map list, always try and read the load base
using the memory functions for local unwinds. For remote unwinds,
if we can't map in the elf image, then try and use the memory
functions instead.

Bug: 21381155
(cherry picked from commit 849a547f)

Change-Id: I96114e296ab1a0f7bfd51b3def1e2da90188249a