[mlir][nvvm] Expose MLIR_NVPTXCOMPILER_ENABLED in mlir-config.h. (#84007)

This is another follow-up of #83004, which made the same change for
`MLIR_CUDA_CONVERSIONS_ENABLED`. As the previous PR, this PR commit
exposes mentioned CMake variable through `mlir-config.h` and uses the
macro that is introduced with the same name. This replaces the macro
`MLIR_NVPTXCOMPILER_ENABLED`, which the CMake files previously defined
manually.