core.internal.backtrace.libunwind

There are two available libunwind: The "upstream" one, inherited from HP, which is maintained as a GNU project, and the LLVM one, part of llvm-project, and the default on Mac OSX. They are both essential part of other languages ABI, and are available in both GCC and LLVM. However, in GCC, only the higher-level functions are exposed (e.g. _Unwind_*) while LLVM expose the higher-level and lower-level (unw_*) functions. Many distributions have a libunwind package as well, that provides the unw_* functions, but since it also supports remote unwinding, the function names are actually platform dependent and binding them is a pain as many things rely on #define. In the future, we would like to implement backtrace using only the higher-level functions (_Unwind_*), which will allow us to not use backtrace and friends directly, and only retrieve the functions names when needed (currently we need to eagerly get the functions names).