Add Portable MatrixFree initialization for GPU builds

[kobayashi1109]

Summary

This PR adds the initial dealii::Portable::MatrixFree data structures
required for the GPU implementation of PRISMS-PF.

For GPU builds, Portable MatrixFree objects are created for each field
and each mesh level and initialized using the existing mapping,
DoFHandler, constraints, quadrature, and update flags.

This work corresponds to the "Define a Portable::MatrixFree object for
GPU data" step and is part of #517.

Validation

The changes were tested on a Tesla V100 with CUDA 12.0 and deal.II 9.6.

• PRISMS-PF builds successfully with PRISMS_PF_GPU=ON.
• The Laplace example links and runs to completion.
• Temporary runtime assertions verified that the Portable MatrixFree
  objects have the expected DoF counts and valid partitioner sizes.

The temporary assertions were removed after validation.

Current limitation

This PR only prepares and initializes the Portable MatrixFree data
structures.

The actual matrix-free operator still uses the existing regular
dealii::MatrixFree path. Therefore, the successful Laplace run does
not yet mean that the Laplace operator itself is executed on the GPU.

Connecting the Portable MatrixFree objects to the operator and using
dealii::Portable::FEEvaluation will be handled in a subsequent change.

[fractalsbyx]

The branch you worked on was 5 months old, so I had to rebase.

[kobayashi1109]

Oh, I'm sorry. I will check that 2026年10月1日(木) 18:20 Xander ***@***.***>:

…

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