bug#19111: 25.0.50; 32 bits temacs.exe is linked with wrong image-base when built on 64 bit Windows host

[Dani Moncayo]

> The s/target/host part is ok. Moving $canonical to the end of the
> mingw-specific code makes little sense. Overwriting it inside the
> mingw-specific block is ok.

My intent is to keep consistency among $canonical, $host and
$configuration (which is not guaranteed currently).

IOW: what we actually want to determine is the host platform;
therefore, IMO it makes more sense to first get the correct value of
$host and then set $canonical and $configuration (which are based on
$host).

-- 
Dani Moncayo