Add powerpc64le VSX coverage in CI

[Copilot]

CI lacked powerpc64 VSX coverage and needed to exercise those code paths under QEMU. This adds a powerpc64le target to the test matrix and ensures VSX-specific flags are used during generated tests.

• CI matrix + QEMU
  • add powerpc64le target and install QEMU (ppc64le) before cross tests
• Test generation
  • include powerpc64le mapping and VSX rustflags in scripts/testgen.py

Example:

- if: matrix.target == 'powerpc64le-unknown-linux-gnu'
  uses: docker/setup-qemu-action@v3
  with:
    platforms: ppc64le

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[Nugine]

Related:

• feat: add POWER ISA 3.0 (VSX/Altivec) SIMD support for base64-simd #60

[Copilot]

Pull request overview

Adds CI coverage for PowerPC64LE (ppc64le) so VSX-related code paths are exercised under QEMU via the existing cross-driven test generation workflow.

Changes:

• Extend scripts/testgen.py with a powerpc mode, mapping powerpc64le-unknown-linux-gnu and applying -C target-feature=+vsx.
• Add powerpc64le-unknown-linux-gnu to the GitHub Actions test matrix.
• Install QEMU user emulation (binfmt) for ppc64le in CI when running that target.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
scripts/testgen.py	Adds powerpc64le target mapping and VSX rustflags so generated tests exercise VSX paths.
.github/workflows/ci.yml	Adds powerpc64le to the matrix and sets up QEMU for that target before running generated cross tests.

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