The syscall unwinding broke since we started connecting syscall frames together. @iamahuman however found a nice feature of GDB which lets us register custom unwinder which is able to handle the syscall frame again and provide connected backtraces once again.
I added such a script in `tools/gdbunwind.py`, which simply needs to be `source`d from `~/.gdbinit` and which will handle unwinding of `__wine_syscall_dispatcher` and `__wine_unix_call_dispatcher` frames.
(In theory it should also be able to connect user frames to unix frames, through KeUserModeCallback, but it then hits the GDB inner call frame check and GDB refuses to continue unwinding. This doesn't look possible to fix without a (probably simple, just to let Python unwinder mark some frames as signal frames) patch on GDB side.)