This adds a gn flag (--backtrace, --no-backtrace) that defaults to
enabling backtraces, which drives a gn variable `enable_backtrace` which
is defaulted true for debug builds.
Backtrace collection is supported on Windows, and on POSIX-like
operating systems via execinfo.h. execinfo support exists in Android,
macOS/iOS, and in glibc and uclibc on Linux. musl libc notably does not
include execinfo support, so this provides an escape hatch to build with
backtrace_stub.cc for situations in which compile time support doesn't
exist.
Proposed as an alternative to
https://github.com/flutter/engine/pull/40958 by @selfisekai.
Issue: https://github.com/flutter/flutter/issues/124285
3bf49d72e4