xbps-src: use default hostdir if unset; added support for x86 chroots ootb.

- If -H (hostdir) is unset, it now defaults to xbps-packages/hostdir. This way
  a single hostdir can be shared by multiple masterdirs without any tweak.

- If bootstrap pkg are installed via 'binary-bootstrap arch', add support to
  reexec xbps-src with linux32 to allow 32bit builds without any tweak or
  additional commands.