[lld] [RISCV] Disable gp relaxation if part of object unreachable (PR #72655)

[Fangrui Song via llvm-commits]

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+# REQUIRES: riscv
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MaskRay wrote:

Do we need `lld/test/ELF/riscv-relax-gp-*` files? Can they be folded into two files or even one?

https://github.com/llvm/llvm-project/pull/72655