Update Python and Clang on x86 dist images

[nikic]

LLVM 12 no longer builds with Python 2, so install Python 3 in
preparation for the upgrade (#81451).

However, Clang 10 does not build with Python 3, so we need update
to Clang 11 as well, which supports both.

Unfortunately, doing so results in errors while linking the
libLLVM.so into other binaries:

__morestack: invalid needed version 2

This is fixed by using LLD instead. Possibly this is due to a binutils
linker bug, but updating to the latest binutils version does not fix
it.

r? @Mark-Simulacrum
cc @cuviper

[Mark-Simulacrum]

I think the changes here shouldn't impact end users at all. We should make sure that LLVM will still build fine without lld/super-recent clang with more modern toolchains (e.g., on Ubuntu 20.04 LTS with roughly just build-essential), but I presume that's checked upstream to some extent too.

r=me if I'm correct about the above.

[cuviper]

FWIW, there was also a case of the __morestack issue with clang and LLVM 11 here:
https://zulip-archive.rust-lang.org/131828tcompiler/25509morestackerrorwhenbuildinglibrustcdriver.html

[nikic]

Yeah, so the linker problem here isn't related to LLVM 12, it also occurs with LLVM 11 and is somehow related to use of Clang 11 and the BFD linker (and possibly some environment factors -- who knows with this old distro) to compile libLLVM.so and then link it.

LLVM 12 itself does build fine (and link into rustc) on Ubuntu 20.04 with a standard toolchain, as that's the system I'm on.

[Mark-Simulacrum]

@bors r+ rollup=never

[bors]

📌 Commit e066dea has been approved by Mark-Simulacrum

[nikic]

I've switched the LLVM_ENABLE_LLD use when llvm.thin-lto=true is set to use LLVM_USE_LINKER=lld instead, as otherwise one can't use llvm.thin-lto=true and llvm.use-linker=lld at the same time.

Missed this previously as I did not delete obj/ between rebuilds.

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit a84ff2b has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit a84ff2b with merge 800ff69df656ec8d4a240efe8fe8c7589424393c...

[bors]

💥 Test timed out

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[Aaron1011]

@bors retry

[bors]

⌛ Testing commit a84ff2b with merge cb6787a...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing cb6787a to master...

[RalfJung]

Looks like this PR broke the produced toolchains: #81489