glibc: re-enable stripping in cross builds

References to the host toolchain are leaking through debug symbols in
glibc, causing gnu cross-builds to always depend on the host toolchain.
The decision to not strip was made in 2012 in order to improve GNU/Hurd
support, and I suspect the reasons that justified it back then do not
apply anymore in 2019.

Closure size before:
/nix/store/v5pxj0bgg627hic2khk4d43z6cjp5v7d-hello-2.10-armv7l-unknown-linux-gnueabihf                            596.8M

After:
/nix/store/llp1ncmpar406rc2vhj7g5ix4yqwna3n-hello-2.10-armv7l-unknown-linux-gnueabihf     23.6M

pkgs/development/libraries/glibc/common.nix

@@ -214,10 +214,6 @@ stdenv.mkDerivation ({
 // lib.optionalAttrs (stdenv.hostPlatform != stdenv.buildPlatform) {
   preInstall = null; # clobber the native hook
 
-  dontStrip = true;
-
-  separateDebugInfo = false; # this is currently broken for crossDrv
-
   # To avoid a dependency on the build system 'bash'.
   preFixup = ''
     rm -f $bin/bin/{ldd,tzselect,catchsegv,xtrace}