[lldb] Stop passing both i386 and i686 in parallel as architectures on Windows

When an object file returns multiple architectures, it is treated
as a fat binary - which really isn't the case of i386 vs i686 where
the object file actually has one architecture.

This allows getting rid of hardcoded architecture triples in
PlatformWindows.

The parallel i386 and i686 architecture strings stem from
5e6f4520 / D7120 and
ad587ae4 / D4658.

Differential Revision: https://reviews.llvm.org/D128617