Function preadv2

pub fn preadv2(
    fd: BorrowedFd<'_>,
    iovs: &mut [IoBufMut<'_>],
    offset: off_t,
    flags: c_int,
) -> ssize_t

Wrapper around the preadv2 syscall for standard I/O slices.

We invoke the Linux syscall directly using libc::syscall instead of libc::preadv2 because some host build environments (e.g. older glibc sysroots like glibc 2.17 used in Android prebuilts for Cuttlefish) do not export preadv2 in libc.