Consume "\nChild exited with status %d\n"

Under certain conditions, GDB emits this to stderr of the process being debugged, which wouldn't normally show up.

We can have some special-casing in the buffering code that removes this last bit of content from the buffer when it hits EOF, so that the user doesn't see this.

https://github.com/bminor/binutils-gdb/blob/4258df85f13dbaf9a1cb5b523bf2b9d390b25e69/gdbserver/target.cc#L178-L188