libc::prctl and the prctl definitions in glibc, musl, and the kernel
headers are C variadic functions. Therefore, all the arguments (except
for the first) are untyped. It is only the Linux man page which says
that prctl takes 4 unsigned long arguments. I have no idea why it says
this.

In any case, the upshot is that we don't need to cast the pointer to an
integer and confuse Miri.