The libbacktrace sources here are synced from:
https://github.com/ianlancetaylor/libbacktrace/

This directory keeps a V-specific amalgamation of upstream commit
`b9e40069c0b47a722286b94eb5231f7f05c08713`.

`backtrace.c` is the local wrapper that defines the static config and
includes `base.c` plus the platform-specific `linux.c`, `darwin.c`, or
`windows.c` amalgamation.