tools/nolibc: align sys_vfork() with sys_fork()

tools/nolibc: align sys_vfork() with sys_fork()

Currently the generic variants of sys_fork() and sys_vfork() differ in
both they precedence of used system calls and the usage of sys_clone()
vs sys_clone3(). While the interface of clone3() in sys_vfork() is more
consistent over different architectures, qemu-user does not support it,
making testing harder. We already handle the different clone()
interfaces for sys_fork() in the architecture-specific headers, and can
do so also for sys_vfork(). In fact SPARC already has such handling and
only s390 is currently missing.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260104-nolibc-vfork-v1-1-a63464b9e4e6@weissschuh.net