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

pkgs/development/compilers/llvm/7/llvm.nix

@@ -17,6 +17,7 @@
 , enableSharedLibraries ? true
 , enablePFM ? !(stdenv.isDarwin
   || stdenv.isAarch64 # broken for Ampere eMAG 8180 (c2.large.arm on Packet) #56245
+  || stdenv.isAarch32 # broken for the armv7l builder
   )
 , enablePolly ? false
 }:

pkgs/development/compilers/llvm/8/llvm.nix

@@ -16,6 +16,7 @@
 , enableSharedLibraries ? true
 , enablePFM ? !(stdenv.isDarwin
   || stdenv.isAarch64 # broken for Ampere eMAG 8180 (c2.large.arm on Packet) #56245
+  || stdenv.isAarch32 # broken for the armv7l builder
 )
 , enablePolly ? false
 }:

pkgs/development/compilers/llvm/9/llvm.nix

@@ -16,6 +16,7 @@
 , enableSharedLibraries ? true
 , enablePFM ? !(stdenv.isDarwin
   || stdenv.isAarch64 # broken for Ampere eMAG 8180 (c2.large.arm on Packet) #56245
+  || stdenv.isAarch32 # broken for the armv7l builder
 )
 , enablePolly ? false
 }: