Another round of toolchain/platform improvements

[dzbarsky]

Changes are visible to end-users: no

Test plan

existing tests

[dzbarsky]

i also bumped your compilers for ya while i'm here

[aspect-workflows]

Bazel 8 (Test)

⚠️ Buildkite build #2654 failed.

---

Bazel 9 (Test)

⚠️ Buildkite build #2654 failed.

---

Bazel 8 (Test)

e2e

⚠️ Buildkite build #2654 failed.

---

Bazel 9 (Test)

e2e

⚠️ Buildkite build #2654 failed.

---

Bazel 8 (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 444ms.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ dzbarsky
❌ aspect-marvin
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[arrdem]

Blocked on e2e test regression.

[dzbarsky]

Blocked on e2e test regression.

yeah, it's because the libc constraints you define are different than the @llvm ones. I'll pick this up again after we merge the @platforms_contrib shared ones