linux: Add recent clone/clone3 and missing signalfd flags.
linux: Add recent clone/clone3 and missing signalfd flags.

The following flags have been introduced:
- CLONE_PIDFD (>=5.2)
- CLONE_INTO_CGROUP (>=5.7)
- SFD_CLOEXEC and SFD_NONBLOCK (>=2.6.27)

andrewrk/zig,andrewrk/zig