glibc: Include a copy of libgcc_s.so.1

This prevents failures like "libgcc_s.so.1 must be installed for
pthread_cancel to work" that occur because Glibc assumes libgcc_s.so.1
to be in Glibc's libdir.

This solution is pretty hacky, because the libgcc_s.so.1 from
bootstrap-tools might be too old. So if we update GCC, programs might
end up using an outdated libgcc_s.so.1. Ideally, we would build
libgcc_s.so.1 *before* Glibc, which might not be impossible...

Fixes #3548.

pkgs/tools/filesystems/squashfs/default.nix

@@ -12,8 +12,6 @@ stdenv.mkDerivation rec {
   buildInputs = [ zlib xz ];
 
   preBuild = "cd squashfs-tools";
-  
-  NIX_LDFLAGS = "-lgcc_s"; # for pthread_cancel
 
   installFlags = "INSTALL_DIR=\${out}/bin";