Wrap linux specific userfaultfd syscall

Wrap linux specific userfaultfd syscall

userfaultfd takes a flags argument and returns a file descriptor.
It shows up in the Linux Test Project syscalls tests as unhandled.

Declare a sys_userfaultfd wrapper in priv_syswrap-linux.h and hook it
for {amd64,arm,arm64,mips64,nanomips,ppc32,ppc64,riscv64,s390x,x86}-linux
using LINXY with PRE/POST handlers in syswrap-linux.c.

Define __NR_userfaultfd in vki-scnums-s390x-linux.h. It was already
defined for all other arches.

https://bugs.kde.org/show_bug.cgi?id=501365