Update windows-gnu targets to set `DebuginfoKind::DWARF`
These targets have always used DWARF debuginfo and not CodeView/PDB debuginfo like the MSVC Windows targets. However, their target definitions claim to use `DebuginfoKind::PDB` probably to ensure that we do not try to allow the use of split-DWARF debuginfo.
This does not appear to be necessary since the targets set their supported split debug info to `Off`. I've looked at all of the uses of these properties and this patch does not appear to cause any functional changes in compiler behavior. I also added UI tests to attempt to validate there is no change in the behavior of these options on stable compilers.
cc ````@mati865```` since you mentioned this in #135739
cc ````@davidtwco```` for split-dwarf
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