[lldb] Fix reading i686-windows executables with GNU environment

25c8a061 / D127048 added an option
for setting the ABI to GNU.

When an object file is loaded, there's only minimal verification
done for the architecture spec set for it, if the object file only
provides one.

However, for i386 object files, the PECOFF object file plugin
provides two architectures, i386-pc-windows and i686-pc-windows.
This picks a totally different codepath in
TargetList::CreateTargetInternal, where it's treated as a fat
binary. This goes through more verifications to see if the
architectures provided by the object file matches what the
platform plugin supports.

The PlatformWindows() constructor explicitly adds the
"i386-pc-windows" and "i686-pc-windows" architectures (even when
running on other architectures), which allows this "fat binary
verification" to succeed for the i386 object files that provide
two architectures.

However, after that commit, if the object file is advertised with
the different environment (either when lldb is built in a mingw
environment, or if that setting is set), the fat binary validation
won't accept the file any longer.

Update ArchSpec::IsEqualTo with more logic for the Windows use
cases; mismatching vendors is not an issue (they don't have any
practical effect on Windows), and GNU and MSVC environments are
compatible to the point that PlatformWindows can handle object
files for both environments/ABIs.

As a separate path forward, one could also consider to stop returning
two architecture specs from ObjectFilePECOFF::GetModuleSpecifications
for i386 files.

Differential Revision: https://reviews.llvm.org/D128268