[stdlib] Back out recent Musl changes for output/input streams

[finagolfin]

Recent pulls #89891 and #90679 tried to improve the async typing for Musl but broke the static linux SDK CI, so temporarily reverting.

[finagolfin]

Temporarily backing out this change to get static linux SDK CI green till @JacobHearst, @ktoso, and @al45tair can figure out a fix. The new test they added built fine locally anyway with the last July 11 trunk snapshot of the static linux SDK, but this is where a static linux PR CI would really come in handy, @justice-adams-apple.

@swift-ci smoke test

[finagolfin]

Merging this Musl revert to try and get trunk static linux SDK CI unblocked, though unsure if this will work. 🤞

[finagolfin]

Oh nice, this revert fixed the trunk static linux SDK CI for now, will let you guys hash out how to bring this change back.

[JacobHearst]

@finagolfin is there a good way for me to reproduce these CI builds locally? Or failing that is there a way to manually trigger the static sdk builds for a specific PR?

[finagolfin]

Locally, you can use the same Docker setup the CI uses, but there is currently no way to manually trigger that on the CI (I have asked the CI team about setting that up and will let you know what they say, but that may take awhile, so better to try locally).

[JacobHearst]

I'm getting a bit lost here in the nuances of the various libc implementations and how that factors into the static linux SDK but I think Musl and WASI actually didn't need any changes at all?

Both Musl and WASI declare stdin/out/err as extern FILE *const which comes into Swift as a let so they don't have any sendability issues.

I tested both locally on my macOS machine with a tiny SPM package and Swift 6.3.3's static linux sdk:

Package.swift

// swift-tools-version: 6.3.3
import PackageDescription

let package = Package(
    name: "HelloWorld",
    targets: [
        .executableTarget(
            name: "HelloWorld",
            swiftSettings: [.swiftLanguageMode(.v6)]
        )
    ],
    swiftLanguageModes: [.v6],
)

main.swift

import Foundation
fputs("Hello world", stderr)

Build Musl: rm -rf .build && swift build --swift-sdk swift-6.3.3-RELEASE_static-linux-0.1.0
Build WASI: rm -rf .build && swift build --swift-sdk swift-6.3.3-RELEASE_wasm

[finagolfin]

@ktoso, wdyt? I'll note that I expected the test you all added to fail for Musl after I backed this out, but as I noted last week, it didn't.

[finagolfin]

@JacobHearst, if you can try enabling Konrad's test for the remaining platforms, as we discussed last month, we can discuss any further changes like this needed on a pull you submit.