enter-chroot: provide automatic linux32 usage when needed

this only affects the case when it's used to enter a 32bit chroot
on an x86_64 host.

having to call the command by hand is much too errorprone, as my
latest experiments have shown.

i forgot to call it nearly every second time and had to rebuild
everything again to make sure nothing crept into the build.

(config.guess from autoconf based software is utterly stupid and
 tries to determine the target architecture by calling uname
 rather than reading the machine triple from the compiler using
 -dumpmachine).
update README