Make tweaks: collate generated/*.o files into their own subdirectory, add PIPEFAIL for better error reporting, and simpler regex to select toys/*/*.c list based on NEWTOY/OLDTOY macros.
Two problems: 1) Sometimes toy_exec() needs to re-exec to gain dropped root permissions, 2) shouldn't recurse forever without exec, stack depth increases and we may leak other resources. Limit it to ~5 levels.
Ignore -m option to cpio, it's what we do anyway so it's not an error.