[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.

Author: krzysz00

mlir/include/mlir/Dialect/GPU/Transforms/Passes.td

@@ -178,8 +178,8 @@ def GpuROCDLAttachTarget: Pass<"rocdl-attach-target", ""> {
            /*default=*/"\"\"",
            "Target features.">,
     Option<"abiVersion", "abi", "std::string",
-           /*default=*/"\"400\"",
-           "Optimization level.">,
+           /*default=*/"\"500\"",
+           "ABI version.">,
     Option<"optLevel", "O", "unsigned",
            /*default=*/"2",
            "Optimization level.">,

mlir/include/mlir/Dialect/LLVMIR/ROCDLOps.td

@@ -635,7 +635,9 @@ def ROCDL_TargettAttr :
     StringRefParameter<"Target triple.", "\"amdgcn-amd-amdhsa\"">:$triple,
     StringRefParameter<"Target chip.", "\"gfx900\"">:$chip,
     StringRefParameter<"Target chip features.", "\"\"">:$features,
-    StringRefParameter<"ABI version.", "\"400\"">:$abi,
+    // Also update the default builder below and rocdl-attach-target in
+    // Dialect/GPU/Transforms/Passes.td .
+    StringRefParameter<"ABI version.", "\"500\"">:$abi,
     OptionalParameter<"DictionaryAttr", "Target specific flags.">:$flags,
     OptionalParameter<"ArrayAttr", "Files to link to the LLVM module.">:$link
   );
@@ -647,7 +649,7 @@ def ROCDL_TargettAttr :
                      CArg<"StringRef", "\"amdgcn-amd-amdhsa\"">:$triple,
                      CArg<"StringRef", "\"gfx900\"">:$chip,
                      CArg<"StringRef", "\"\"">:$features,
-                     CArg<"StringRef", "\"400\"">:$abiVersion,
+                     CArg<"StringRef", "\"500\"">:$abiVersion,
                      CArg<"DictionaryAttr", "nullptr">:$targetFlags,
                      CArg<"ArrayAttr", "nullptr">:$linkFiles), [{
       return Base::get($_ctxt, optLevel, triple, chip, features, abiVersion,