Auto merge of #28593 - gandro:rumprun, r=alexcrichton

This adds a new target, `x86_64-rumprun-netbsd`, and related changes to `std`.

Rumprun is a unikernel platform that provides a POSIX-y interface. For the most part, rumprun uses NetBSD's libc and drivers, therefore `target_os` is `netbsd`. However, being a unikernel, rumprun does not support process management, signals or virtual memory, so related functions might fail at runtime. For this reason, stack guards are disabled in `std`.

To support conditional compilation, `target_env` is set to `rumprun`. Maybe `target_vendor` would be technically more fitting, but it doesn't seem to be worth the hassle.

Code for rumprun is always cross-compiled, it uses always static linking and needs a custom linker. The target makes use of the newly introduced `no_default_libs` flag, as the rumprun linker will otherwise not use the correct search path.

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