rename mingw's i386 target to i686

[waruqi]

#5904

[TurnOffNOD]

i686 means that compiled binary should run at least 686 architecture, and means it drops out suport of architecture older than 686.

i386 means that it still supports architecture older than 686.

[waruqi]

I added a poll option, x86_64, i386, i686

[ifarbod]

I'd say add and rename to i686 and have backward compatibility for i386, x86