unix: add consts for ELF handling

This adds a few constants that help to handle ELF files and coredumps.

Change-Id: Ifd4c1bd4c8839065af4233ccbf42964a8e96fdf8
Reviewed-on: https://go-review.googlesource.com/c/sys/+/713980
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>