Brings in 3e6f63e (drop libexecinfo), aa31cdf (Chimera/musl portability),
bc10810 (Bionic atoi, canary signal exit codes) and cb2e9dd (CI fixes).
Conflicts:
- tests/native_update_test.cc: keep both <cerrno> and <csignal>.
- tests/webview_helper_launcher_test.cc: keep the existing python3 socket
read; it already solves the dash multi-digit fd redirection problem that
upstream fixed by switching the helper to bash.