Add HelenOS support

[mvolfik]

Description

Add support for HelenOS. I would like to get this backported to 0.2, because I also have a stdlib support ready, which depends on libc 0.2. I'll be sending that patch to rust-lang/rust rather soon (this or next week), I hope.

@rustbot label stable-nominated

Sources

All HelenOS source code is at https://github.com/HelenOS/helenos/. There are comments in this patch attributing each section to a specific header file. Files from common/ and abi/ get included in HelenOS libc by the build system there, from uspace/lib we use c, inet and posix, so that's what we link. libstartfiles is HelenOS' version of crt* files, because they don't distribute them along with the compiler toolchain, but instead as a library.

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

Tested locally as cargo test (without the --target flag), testing for HelenOS targets will require support here in ctest2, I hope for a tier3 target, it is ok to go without these tests for now?

[rustbot]

r? @tgross35

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[mvolfik]

Ugh, the only error is missing HelenOS target... I planned to merge the compiler and stdlib support at once. But this makes it seem like I will need to first merge no-std support for HelenOS in compiler, then this libc patch can go through, and only then the stdlib PR makes sense?

[tgross35]

Is there a proposal for adding the target to Rust? The changes look pretty reasonable to me, but getting it at least okayed for rustc is a prerequisite for libc changes.

Ugh, the only error is missing HelenOS target... I planned to merge the compiler and stdlib support at once. But this makes it seem like I will need to first merge no-std support for HelenOS in compiler, then this libc patch can go through, and only then the stdlib PR makes sense?

It is usually preferable to do this anyway; the compiler changes are usually pretty easy and can get merged as soon as the proposal is accepted. Then library changes can build up from there and things don't get slowed down by having a large diff to review at once.