Auto merge of #2257 - sunfishcode:sunfishcode/preadv2-offset, r=JohnTitor
Auto merge of #2257 - sunfishcode:sunfishcode/preadv2-offset, r=JohnTitor

Add `preadv64v2` and `pwritev64v2` for linux-gnu.

These are similar to `preadv2` and `pwritev2` but use `off64_t` instead
of `off_t`.

rust-lang/libc,rust-lang/libc,rust-lang/libc