treewide: Add lots of meta.platforms

Build-tested on x86_64 Linux & Mac.

pkgs/development/libraries/libsigsegv/2.5.nix

@@ -12,6 +12,7 @@ stdenv.mkDerivation rec {
     homepage = http://libsigsegv.sf.net;
     description = "A library for handling page faults in user mode";
     branch = "2.5";
+    platforms = stdenv.lib.platforms.linux;
   };
 
   doCheck = true;

pkgs/development/libraries/libsigsegv/default.nix

@@ -28,5 +28,6 @@ stdenv.mkDerivation rec {
     license = stdenv.lib.licenses.gpl2Plus;
 
     maintainers = [ ];
+    platforms = stdenv.lib.platforms.unix;
   };
 }