[TargetVersion] Only enable on RISC-V and AArch64 (#115991)

Address https://github.com/llvm/llvm-project/issues/115000

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This patch constrains the target_version feature to work only on RISC-V
and AArch64 to prevent crashes in Clang.

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Co-authored-by: Aaron Ballman <aaron@aaronballman.com>