[3.14] GH-136895: Update JIT builds to use LLVM 21 (GH-140329)

[mcepl]

Backport of GH-140329 to 3.14, with the addition of:

• JIT: harden LLVM 20 upgrade with defensive improvements

• Enable x86_64 trampoline support on all x86_64 platforms, not just
  macOS. While currently only macOS emits X86_64_RELOC_BRANCH
  relocations, this ensures the trampoline infrastructure is ready if
  future LLVM changes produce out-of-range calls on other x86_64
  platforms.

• Add error handling to extract_tarball() in get_external.py so that
  corrupted or incomplete downloads produce a clear error message
  instead of an opaque traceback.

• Replace hardcoded LLVM version check in get_externals.bat with
  a prefix-based 'llvm-' match using findstr, so future LLVM version
  bumps only require changing the version string in one place.

Also, bump the JIT toolchain from LLVM 20 to LLVM 21 (21.1.8). Updated across
CI workflows, build scripts, documentation, and the externals tag.

• Issue: Update JIT builds to use LLVM 20 #136895

[Fidget-Spinner]

I don't think we can do this. Changing a build dependency in a patch release of 3.14 is very strange and usually not allowed.

[Fidget-Spinner]

I don't think we can do this. Changing a build dependency in a patch release of 3.14 is very strange and usually not allowed.

Really sorry. Unless there's strong reasons to do so (e.g. security), this isn't possible I think.

[mcepl]

I don't think we can do this. Changing a build dependency in a patch release of 3.14 is very strange and usually not allowed.

Oh well, point taken. Could I at least ask for the review of the patch (partciularly

[chris-eibl]

Could I at least ask for the review of the patch

If you care about Windows, I'd consider including #142052.
But as @Fidget-Spinner said, updating LLVM in 3.14 is too big of a change.

[mcepl]

On Fri Mar 27, 2026 at 8:47 AM CET, Chris Eibl wrote: If you care about Windows, I'd consider including #142052.

I am a maintainer of Python for SUSE; no, I don’t care about Windows at all, I was trying to fix those GitHub Actions just so that this PR would be acceptable.