llvm_7,8,9: disable libpfm on aarch32

The armv7l builder has has the same issue some of the aarch64 ones have (#56245)

See:
https://hydra.nixos.org/eval/1583749?filter=llvm_&compare=1583176&full=#tabs-still-fail
https://nix-cache.s3.amazonaws.com/log/3ppqyssyqj5d6sh257zfb5rb6km4m6sa-llvm-7.1.0.drv
https://nix-cache.s3.amazonaws.com/log/rmacd5xwkw3lyb6zknc3sgdcb3cvk6k1-llvm-8.0.1.drv
https://nix-cache.s3.amazonaws.com/log/6s8pasqwkrzzv5lgw1l22zy7mrpzx2p3-llvm-9.0.1.drv