[mlir][gpu] Replace MLIR_GPU_TO_HSACO_PASS_ENABLE by more generic one. (#84001)

This is another follow-up of #83004. The PR replaces the macro
`MLIR_GPU_TO_HSACO_PASS_ENABLE` with the more generic macro
`MLIR_ENABLE_ROCM_CONVERSIONS`. Until now, the former has been defined
if and only if the latter evaluated to true in CMake. However, the
former was not defined when the latter evaluated to false, in which case
a warning was raised if compiled with `-Wundef`. Using a single macro
relies on the `#cmakedefine01` mechanism that ensures the macro is
always set to either 0 or 1.