[mlir][spirv] Change Spirv.GlobalVariableOp builtin -> built_in (#152148)
GlobalVariableOp describes that built_in specifies SPIR-V BuiltIn
decoration associated with the op. The attribute was defined as builtin
in the tablegen (no uderscore). This was causing correct
GlobalVariableOp decorations like: built_in("GlobalInvocationId") to be
saved as a new attribute making it impossible to access the built_in
attribute through getBuiltinAttr.