More libbacktrace fixes.

Included in minor fix ups is the addition of a warning macro to replace
all of the ALOGW calls.

Fix a race where multiple threads could be attempting to unwind the threads
of the current process at the same time.

Bug: 8410085

Change-Id: I02a65dc778dde690e5f95fc8ff069a32d0832fd1
8 files changed