build: bump LLVM requirement to 21

[aduh95]

simdjson 5.0.0 (bot PR is #66495) has introduced use of exceptions (on methods we don't use), and clang 19 refuses to compile it with -fno-exceptions (which we pass). clang 21 accepts to compile as long as the throw statements can be tree-shaken away, so bumping our requirement on Node.js 27+ might be preferable than floating a patch on simdjson.

/cc @nodejs/build

[nodejs-github-bot]

Review requested:

• @nodejs/actions
• @nodejs/build
• @nodejs/tsc

[richardlau]

I don't think anything in the Jenkins CI is on clang 21, other than maybe fedora-latest (although testing on that is currently suspended pending resolution of the DigitalOcean situation) and rhel10. Would have to check if we'd need to be on Ubuntu 26.04 (which we currently are not running) instead of 24.04 for clang 21.

We generally avoid compiling our own version of e.g. clang or install a version on a Linux distro outside of the distro's package manager because it does not replicate what we'd be expecting contributors/downstream builders to be doing.