Cross-compiling win32 & other targets

Hi! (sorry for previous base64+html)

I've recently reworked my environment used for building windows
binaries without ever touching windows: where cross mingw toolchain
compiles the C parts, native linux sbcl is used as a host, and it all
results in a windows binary.
 
The new approach simplified my building to the point of not needing
binfmt_misc, so it runs in unprivileged containers or just on any
system with wine >= 6.x and a fresh mingw. Two parts:
 
* https://github.com/sbcl/sbcl/pull/90 hooks $SBCL_RUNNER into the
  build process wherever a host binary wants to call a target binary

* https://github.com/akovalenko/wrun provides several examples of
  using this hook (targeting windows, aarch64 android, aarch64 linux)
 
I hope greatly that the SBCL_RUNNER part will eventually find a way to
the upstream.