Fix handling of libs with a ro map and a rx map.

When the linker was changed so that it put a read-only map and
read-execute map, this code incorrectly computed the relative pc and
offset. Modify to look for the actual start map in this case.

Also, change the load_base name to load_bias.

Bug: 120613266

Test: Dumped logcat while dumping backtraces.

Change-Id: I6628694c5222ea34d63217af3d138707d4900004