[Chapel Merge] Additional updates for LLVM 21

Branch: refs/heads/main
Revision: 98f1ef85bcef34e4fb00e66c4661eac276de7d1a
Author: jabraham17
Link: Additional updates for LLVM 21 by jabraham17 · Pull Request #28002 · chapel-lang/chapel · GitHub
Log Message:
Additional updates for LLVM 21 (#28002)

Additional updates for Chapel to support LLVM 21

The core support was added in
Add support for LLVM 21 by jabraham17 · Pull Request #27941 · chapel-lang/chapel · GitHub, this PR makes some
portability adjustments and adds a test config

[Reviewed by @arifthpe]

Compare: Comparing 61a4522485d4185f417ceb6873b04317f89470fd...394c144c0a8a21f80b97fbad8a3c58831ebb7448 · chapel-lang/chapel · GitHub

Diff:
M doc/rst/usingchapel/prereqs.rst
A util/cron/test-linux64-llvm21.bash
M util/devel/test/portability/apptainer/current/arch/image.def
M util/devel/test/portability/apptainer/current/centos-stream-10/image.def
M util/devel/test/portability/apptainer/current/centos-stream-9/image.def
M util/devel/test/portability/apptainer/current/fedora-43/image.def
M util/devel/test/portability/provision-scripts/apt-get-and-linuxbrew.sh
D util/devel/test/portability/provision-scripts/pacman-llvm20.sh
M util/packaging/homebrew/chapel-main.rb
M util/packaging/rpm/fc43/Dockerfile.template
M util/packaging/rpm/fc43/spec.template
https://github.com/chapel-lang/chapel/pull/28002.diff