Update Arm libcxx runners' compiler from clang 19 to clang 20 or 21

[DavidSpickett]

Since clang 21 released, libcxx will be dropping support for clang 19 soon. Linaro's Arm runners are still on clang 19.1.7.

We (Linaro) plan to update to clang 21.1.1 and have tested this with libc++ and our other buildbots without issue.

What remains is to check that our internal CI won't be affected, we are doing this right now.

[DavidSpickett]

@philnik777 FYI that this is being worked on.

[llvmbot]

@llvm/issue-subscribers-infrastructure

Author: David Spickett (DavidSpickett)

Since clang 21 released, libcxx will be dropping support for clang 19 soon. Linaro's Arm runners are still on clang 19.1.7.

We (Linaro) plan to update to clang 21.1.1 and have tested this with libc++ and our other buildbots without issue.

What remains is to check that our internal CI won't be affected, we are doing this right now.

[DavidSpickett]

Our builders are now using clang 21.1.1.