CI: ensure more tests are run using LLVM

In !11902 (closed), I found out that there were no jobs in full-ci that would run the FMA_Primops test using LLVM, even though the test specifies when(have_llvm(), extra_ways(["optllvm"])). I had to tag the MR with LLVM backend to get the x86_64-linux-deb12-validate+llvm job which ran that job using LLVM. Moreover, it seems that the aarch64-linux-deb12-validate+llvm did not run that test with LLVM (despite the job name), as evidenced by the fact that this job passed at a commit of the MR which is known to be wrong (as evidenced by this job failing).