hurd: Fix build after __pread64 usage in the dynamic loader

Commit 95c10569 ("elf: Use nocancel
pread64() instead of lseek()+read()") added calls to __pread64 to
the dynamic loader.  On Hurd, this needs an implementation in the
dynamic loader because the rtld-pread64 rebuild pulls in too many
symbols.

Fixes: 95c10569
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>