tools/nolibc: Provide vfork()

To allow testing of vfork() support in the arm64 basic-gcs test provide an
implementation for nolibc, using the vfork() syscall if one is available
and otherwise clone3(). We implement in terms of clone3() since the order
of the arguments for clone() varies between architectures.

As for fork() SPARC returns the parent PID rather than 0 in the child
for vfork() so needs custom handling.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250703-arm64-gcs-vfork-exit-v3-2-1e9a9d2ddbbe@kernel.org


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>