Implement `//@ needs-target-std` compiletest directive
Closesrust-lang/rust#141863.
Needed to unblock rust-lang/rust#139244 and rust-lang/rust#141856.
### Summary
This PR implements a `//@ needs-target-std` compiletest directive that gates test execution based on whether the target supports std or not. For some cases, this should be preferred over e.g. some combination of `//@ ignore-none`, `//@ ignore-nvptx` and more[^none-limit].
### Implementation limitation
Unfortunately, since there is currently [no reliable way to determine from metadata whether a given target supports std or not](https://github.com/rust-lang/rust/issues/142296), we have to resort to a hack. Bootstrap currently determines whether or not a target supports std by a naive target tuple substring comparison: a target supports std if its target tuple does *not* contain one of `["-none", "nvptx", "switch"]` substrings. This PR simply pulls that hack out into `build_helpers` to avoid reimplementing the same hack in compiletest, and uses that logic to inform `//@ needs-target-std`.
### Auxiliary changes
This PR additionally changes a few run-make tests to use `//@ needs-target-std` over an inconsistent combination of target-based `ignore`s. This should help with rust-lang/rust#139244.
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r? bootstrap
[^none-limit]: Notably, `target_os = "none"` is **not** a sufficient condition for "target does not support std"
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