[mlir][AMDGPU] Actually update the default ABI version, add comments (#79185)

Much confusion occurred earlier today when updating the fallback `int
abi;` in addControlVariables() didn't do anything. THis was because that
that value is the fallback for if the ABI version fails to parse ...
which it always should, because it has a default value that comes from
multiple different places.

This commit updates all the places said default variable can come from,
namely:
1. The ROCDL target attribute definition
2. The ROCDL target attribute's builders
3. The rocdl-attach-target pass's default option values.

With this, the printf test is passing.