uclibc fix kernel constant tests + docker ci build fixes

[skrap]

Description

Today's uClibc builds in CI cannot test many recently-added linux kernel features, as the prebuilt bootlin toolchains they rely on are built against kernel 5.15.

This changes the ci/install-uclibc.sh script to use a toolchain provided via an environment variable. This allows projects and developers who want to run the uclibc tests to build based on kernel 7.0 headers, and use that toolchain for running the libc build and test in CI.

Best way to test is via:

UCLIBC_TOOLCHAIN_URL=https://github.com/skrap/libc-downstream-ci/releases/download/v0.1-toolchain/arm-buildroot-linux-uclibcgnueabihf_sdk-buildroot.tar.gz  \
LIBC_CI_ZBUILD_STD=1 \
TEST_UCLIBC_TIME64=1 \
RUSTUP_TOOLCHAIN=nightly \
./ci/run-docker.sh armv7-unknown-linux-uclibceabihf

NOTE: the toolchain URL provided above is from my github account and shouldn't be directly included in the source, as I'm sure I would get dinged for bandwidth usage. It's my expectation that developers would build their own or ask nicely to use mine in a responsible way.

Abandoned task: building a toolchain

My first attempt at this PR included building a toolchain from source via buildroot, but that process takes longer than the github action timeout (which seems to be 20 minutes) so I abandoned that approach.

Results

With this change, it's now possible for me to build the libc in a fork of the libc-downstream-ci example posted by @tgross35. See here for example.

Sources

N/A

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

[skrap]

@rustbot review

[tgross35]

What is involved in preparing the SDK that gets downloaded? I think the current setup is fine enough, but if it's not too bad then I feel like the docker container may as well just build it. Or build it as part of run-docker.sh and copy it into the container to make caching easier.

View changes since this review

[tgross35]

"uclibc: move to configurable external prebuilt toolchain": maybe just export RUSTFLAGS="${RUSTFLAGS:-}" at the top of run() so we only need that in one place

[tgross35]

"uclibc: move to configurable external prebuilt toolchain": missing trailing \n

[tgross35]

I also think it would be fine to link to your GH URL since libc's frequent CI won't be needing it. But for that, ideally the script should be checked into your repo somewhere (or build+release via GHA?) and the sha could be checked here so it's easy to reproduce.

[tgross35]

Also no problem of course but just a heads up, looks like the email you used here isn't linked to your GH account (e.g. https://github.com/rust-lang/libc/commit/3156347e405a3dcb48f1b639ed420681ea897745.patch).