Abstract windows toolchain setup

This commit splits up the windows toolchain setup logic into two
functions.

* FP_INSTALL_WINDOWS_TOOLCHAIN - deals with downloading the toolchain if
  it isn't already downloaded
* FP_SETUP_WINDOWS_TOOLCHAIN - sets the environment variables to point
  to the correct place

FP_SETUP_WINDOWS_TOOLCHAIN is abstracted from the location of the mingw
toolchain and also the eventual location where we will install the
toolchain in the installed bindist.

This is the first step towards #23608