msrtool: Make the tool available only on x86 Linux

On Linux this tool reads Model Specific Registers (MSRs) from
/dev/cpu/<cpu_num>/msr, a device implemented exclusively for x86
instruction set CPUs. It doesn't make sense to offer it on other
platforms.

pkgs/tools/misc/coreboot-utils/default.nix

@@ -38,6 +38,7 @@ let
     msrtool = generic {
       pname = "msrtool";
       meta.description = "Dump chipset-specific MSR registers";
+      meta.platforms = [ "x86_64-linux" "i686-linux" ];
       buildInputs = [ pciutils zlib ];
       preConfigure = "export INSTALL=install";
     };