Auto merge of #134765 - Noratrieb:linux-none-cant-unwind-silly, r=jieyouxu

Improve default target options for x86_64-unknown-linux-none

Without a standard library, we cannot unwind, so it should be panic=abort by default.

Additionally, it does not have std because while it is Linux, it cannot use libc, which std uses today for Linux.

Using PIE by default may be surprising to users, as shown in #134763, so I've documented it explicitly. I'm not sure if we want to count that as fixing the issue or not.

cc `@morr0ne,` as you added the target (and are the maintainer), and `@Noratrieb,` who reviewed that PR (:D).

Author: bors