treewide: use isx86 where appropriate

nixos/modules/installer/cd-dvd/iso-image.nix

···
 
     isoImage.squashfsCompression = mkOption {
       default = with pkgs.stdenv.targetPlatform; "xz -Xdict-size 100% "
-                + lib.optionalString (isx86_32 || isx86_64) "-Xbcj x86"
+                + lib.optionalString isx86 "-Xbcj x86"
                 # Untested but should also reduce size for these platforms
                 + lib.optionalString isAarch "-Xbcj arm"
                 + lib.optionalString (isPower && is32bit && isBigEndian) "-Xbcj powerpc"

pkgs/applications/networking/instant-messengers/jami/daemon.nix

···
       configureFlags = old.configureFlags
         ++ (readLinesToList ./config/ffmpeg_args_common)
         ++ lib.optionals stdenv.isLinux (readLinesToList ./config/ffmpeg_args_linux)
-        ++ lib.optionals (stdenv.isx86_32 || stdenv.isx86_64) (readLinesToList ./config/ffmpeg_args_x86);
+        ++ lib.optionals stdenv.hostPlatform.isx86 (readLinesToList ./config/ffmpeg_args_x86);
       outputs = [ "out" "doc" ];
       meta = old.meta // {
         # undefined reference to `ff_nlmeans_init_aarch64'

pkgs/applications/video/handbrake/default.nix

···
   ++ optional (!useGtk) "--disable-gtk"
   ++ optional useFdk "--enable-fdk-aac"
   ++ optional stdenv.isDarwin "--disable-xcode"
-  ++ optional (stdenv.isx86_32 || stdenv.isx86_64) "--harden";
+  ++ optional stdenv.hostPlatform.isx86 "--harden";
 
   # NOTE: 2018-12-27: Check NixOS HandBrake test if changing
   NIX_LDFLAGS = [ "-lx265" ];

pkgs/games/gemrb/default.nix

···
 let
   # the GLES backend on rpi is untested as I don't have the hardware
   backend =
-    if (stdenv.isx86_32 || stdenv.isx86_64) then "OpenGL" else "GLES";
+    if stdenv.hostPlatform.isx86 then "OpenGL" else "GLES";
 
   withVLC = stdenv.isDarwin;
 

pkgs/os-specific/linux/nmon/default.nix

···
   dontUnpack = true;
   buildPhase = "${stdenv.cc.targetPrefix}cc -o nmon ${src} -g -O2 -D JFS -D GETUSER -Wall -D LARGEMEM -lncurses -lm -g -D ${
     with stdenv.targetPlatform;
-    if isx86_32 || isx86_64 then "X86"
+    if isx86 then "X86"
     else if isAarch then "ARM"
     else if isPower then "POWER"
     else "UNKNOWN"