glibc: expose enableCET as overridable argument, default "permissive"

this should be a gentler way to introduce CET-compiled
binaries into general usage

pkgs/development/libraries/glibc/common.nix

···
 , withLinuxHeaders ? false
 , profilingLibraries ? false
 , withGd ? false
+, enableCET ? false
 , extraBuildInputs ? []
 , extraNativeBuildInputs ? []
 , ...
···
       # and on aarch64 with binutils 2.30 or later.
       # https://sourceware.org/glibc/wiki/PortStatus
       "--enable-static-pie"
-    ] ++ lib.optionals stdenv.hostPlatform.isx86_64 [
+    ] ++ lib.optionals (enableCET != false) [
       # Enable Intel Control-flow Enforcement Technology (CET) support
-      "--enable-cet"
+      "--enable-cet${if builtins.isString enableCET then "=${enableCET}"  else ""}"
     ] ++ lib.optionals withLinuxHeaders [
       "--enable-kernel=3.10.0" # RHEL 7 and derivatives, seems oldest still supported kernel
     ] ++ lib.optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
···
   passthru = { inherit version; minorRelease = version; };
 }
 
-// (removeAttrs args [ "withLinuxHeaders" "withGd" "postInstall" "makeFlags" ]) //
+// (removeAttrs args [ "withLinuxHeaders" "withGd" "enableCET" "postInstall" "makeFlags" ]) //
 
 {
   src = fetchurl {

pkgs/development/libraries/glibc/default.nix

···
 , withLinuxHeaders ? true
 , profilingLibraries ? false
 , withGd ? false
+, enableCET ? if stdenv.hostPlatform.isx86_64 then "permissive" else false
 , pkgsBuildBuild
 , libgcc
 }:
···
 in
 
 (callPackage ./common.nix { inherit stdenv; } {
-  inherit withLinuxHeaders withGd profilingLibraries;
+  inherit withLinuxHeaders withGd profilingLibraries enableCET;
   pname = "glibc" + lib.optionalString withGd "-gd" + lib.optionalString (stdenv.cc.isGNU && libgcc==null) "-nolibgcc";
 }).overrideAttrs(previousAttrs: {