Toolchain: Remove OpenBLAS-based architecture detection

[Growl1234]

Replace OPENBLAS_ARCH with a lightweight host architecture detection based on uname -m, normalizing only known equivalent names such as x86_64/amd64 and aarch64/arm64.

Also remove OPENBLAS_LIBCORE and get_openblas_arch.sh. For native builds, OpenBLAS already performs its own CPU/core detection when TARGET is not specified, while the existing DYNAMIC_ARCH fallback is preserved.

This removes an unnecessary OpenBLAS download/probe from the early toolchain stages and simplifies the architecture handling.

See also cp2k/cp2k#6023.

[Growl1234]

Toolchain Quick Test failed because cmake.org is down.

[Copilot]

🟡 Changes recommended

Preserve the existing MKL i386/ia32 mapping and clarify or restore the changed OpenBLAS fallback behavior.

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Pull request overview

Replaces OpenBLAS-based architecture probing with normalized uname -m detection and simplifies toolchain architecture handling.

Changes:

• Adds SYSTEM_ARCH normalization across toolchain installers.
• Updates OpenBLAS target and fallback behavior.
• Removes obsolete OpenBLAS variables and probe script.

File summaries

File	Summary
toolchain/scripts/stage3/install_elpa.sh	Uses normalized architecture for CPU feature detection.
toolchain/scripts/stage2/install_openblas.sh	Updates target selection and fallback behavior; fallback policy requires clarification.
toolchain/scripts/stage2/install_mkl.sh	Uses SYSTEM_ARCH; currently rejects i386 before the prior ia32 mapping.
toolchain/scripts/stage1/install_openmpi.sh	Uses normalized architecture for compatibility flags.
toolchain/scripts/stage0/setup_buildtools.sh	Removes the OpenBLAS architecture probe.
toolchain/scripts/stage0/install_cmake.sh	Selects CMake artifacts by system architecture.
toolchain/scripts/package_versions.sh	Removes OpenBLAS-dependent checksum selection.
toolchain/scripts/get_openblas_arch.sh	Removes the obsolete probe script.
toolchain/scripts/common_vars.sh	Defines normalized host architecture detection.

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 2
• Review effort level: Lite

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