tools/nolibc: cast pointers returned from system calls through integers

tools/nolibc: cast pointers returned from system calls through integers

Currently all system call wrappers return 'long' integers which can be
directly cast to 'void *' if the returned value is actually a pointer.
An upcoming change will change the system call wrappers to sometimes
return 'long long' which can not be cast to a pointer directly.

Add explicit cast through 'long' to prepare for this.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260418-nolibc-largefile-v1-3-b91f0775bac3@weissschuh.net