llvmPackages: set version 7 for armv6

In later versions the compiler-rt seems to be producing invalid code
and ultimately doesn't build on armv6.

Note: armv6 builds have been broken since bf096238.

pkgs/top-level/all-packages.nix

···
       /**/ if platform.isDarwin then 11
       else if platform.isFreeBSD then 7
       else if platform.isAndroid then 12
+      else if platform.system == "armv6l-linux" then 7  # This fixes armv6 cross-compilation
       else if platform.isLinux then 11
       else if platform.isWasm then 12
       else latest_version;