Remove support for OMF, make MsCOFF be x86 ...

Now that OMF support is being removed, dmd-latest is broken in the CI.
Since DMD no longer supports it, and we are mostly concerned about
supporting the latest 10 releases, we're keeping x86_omf and x86_mscoff
available but making them aliases to x86.

Adapt determinePlatform to the new defaults for Win32 ...

Starting with DMD 2.099, `-m32` defaults to MsCOFF instead of OMF and
`-m32omf` is required to use the OMF format + OPTLINK.

This commit changes the logic to select the appropriate logic based
on the current compiler version. Potentially using the wrong object file
format during platform probing is fine because the codegen is skipped
(dmd is invoked with `-c -o-`).