.cirrus: Add manual amd64 and aarch64 jobs using GCC 14

[imp]

When to cull gcc-12?

[jhb]

In D48290#1101397, @imp wrote:

When to cull gcc-12?

Good question. We currently have 4 versions of LLVM listed. I know we have to revert upstream libc++ commits because LLVM doesn't support more than the most recent GCC version (which is kind of insane for us). I don't think we need more than 3 GCC versions at a time, and would probably be fine with dropping GCC 12 even at this point since I have both stable branches building fine with GCC 14. But I'm not sure what makes sense policy-wise.