Add load base to map for relocation packing.

The new linker relocation packing support uses non-zero load bases,
so we better handle them properly.

Also print out the load base for a map if it's non-zero.

Bug: 20687795
Change-Id: Iec2d1db2051e7b4a278c1dfa57d745128a7f2974
diff --git a/libbacktrace/Backtrace.cpp b/libbacktrace/Backtrace.cpp
index 91ca8b7..d75c210 100644
--- a/libbacktrace/Backtrace.cpp
+++ b/libbacktrace/Backtrace.cpp
@@ -97,12 +97,7 @@
     map_name = "<unknown>";
   }
 
-  uintptr_t relative_pc;
-  if (BacktraceMap::IsValid(frame->map)) {
-    relative_pc = frame->pc - frame->map.start;
-  } else {
-    relative_pc = frame->pc;
-  }
+  uintptr_t relative_pc = BacktraceMap::GetRelativePc(frame->map, frame->pc);
 
   std::string line(StringPrintf("#%02zu pc %" PRIPTR "  %s", frame->num, relative_pc, map_name));
   if (!frame->func_name.empty()) {