Cleanup backtrace setup code

Obtain the list of functions to backtrace from the environment on Android as
well.
Import libbacktrace, sans autotools build files
Add replacement headers for libbacktrace

libbacktrace #include's two header files from GCC.  Provide minimal
replacements for those files.  For filenames.h, declare missing macros
assuming POSIX-style paths.  For dwarf2.h, #include system dwarf.h and
declare missing enum tags and define missing enum values as macros.
Add libbacktrace CMake build script
Add libbacktrace to third-party libs for ELF
Implement libbacktrace provider
Switch to libbacktrace provider
Remove GlibcBacktraceProvider
Use libstdc++ demangler
common: Fix C pre-processor logic.
common: Add copyright headers to trace_backtrace.hpp
Merge branch 'libbacktrace'