Target Intel Clang under Rosetta

[MikeMcQuaid]

• Apple Clang on macOS 26 can select its native arm64 target even while Intel Homebrew runs under Rosetta 2. Superenv then combines that target with -march=westmere, which Clang rejects.
• Detect the translated process using Homebrew's logical and physical processor values.
• Pass -arch x86_64 only to Apple clang and clang++, leaving native ARM builds and alternative compilers unchanged.
• Cover the failing flag combination with a shim regression test.
• Users must still invoke Intel Homebrew with arch -x86_64.

Fixes #23722

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Codex with GPT 5.6-sol at Extra High effort, with local review and testing.

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[krehel]

Thanks!