Rewrite libbacktrace using C++.

The old code was essentially trying to be C++ in C and was awkward. This
change makes it all objects with a thin layer that C code can use.

There is a C++ backtrace object that is not very useful, this code will
replace it.

This change also includes moving the backtrace test to a gtest, and adding
coverage of all major functionality.

Bug: 8410085
Change-Id: Iae0f1b09b3dd60395f71ed66010c1ea5cdd37841