libselinux: fix cross-compilation from x86_32

libselinux incorrectly initializes the host platform's arch using uname -m,
which actually returns the build platform's arch. This results in an invalid
compile flag (-mno-tls-direct-seg-refs) when compiling from x86_32.
So we need to set the host arch variable explicitly.

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

···
     "SHLIBDIR=$(out)/lib"
 
     "LIBSEPOLA=${lib.getLib libsepol}/lib/libsepol.a"
+    "ARCH=${stdenv.hostPlatform.linuxArch}"
   ] ++ optionals enablePython [
     "PYTHON=${python3.pythonForBuild.interpreter}"
     "PYTHONLIBDIR=$(py)/${python3.sitePackages}"