lld: fix Linx HI/LO relocation pairing

[touzicongbupianren]

This PR fixes Linx LLD HI/LO relocation pairing.

Problem:

• Some R_LINX_LO12 relocations directly reference a final symbol with a non-zero addend.
• The previous Linx HI/LO recovery could match by symbol only, which may pair a LO12 with the wrong HI20.
• Linx block layout may also place the matching HI20 after the LO12 in the same input section.

Fix:

• Match R_LINX_PCREL_HI20 by exact (symbol, addend).
• Search both before and after the LO12 in the same input section.
• Keep LO12 calculation based on the paired HI20 target address.

Validation:

• Rebuilt ld.lld.
• glibc G1b relink passed.
• Dynamic Linx printf ELF links with no R_LINX_LO12 warning.

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[gemini-code-assist]

Code Review

This pull request updates the relocation handling in lld/ELF/InputSection.cpp to support recovering R_LINX_PCREL_HI20 relocations in either direction by exact symbol and addend, and defers error reporting for unsupported defined anchors. It also updates getRelocTargetVA to compute the target address directly for RE_LINX_PC_INDIRECT. The reviewer suggested a performance optimization in findLinxPCRelHiBySymbol to avoid calling llvm::lower_bound twice by copying the iterator for the backward search.

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[gemini-code-assist]

We can avoid calling llvm::lower_bound twice by copying the iterator it for the backward search. This eliminates the redundant binary search overhead on the relocation list.

  const Relocation *before = nullptr;
  for (auto rollbackIt = it; rollbackIt != isec->relocs().begin();) {
    --rollbackIt;
    if (rollbackIt->sym == sym && rollbackIt->addend == addend && isLinxPCRelHi(rollbackIt->type)) {
      before = &*rollbackIt;
      break;
    }
  }

  const Relocation *after = nullptr;
  for (; it != isec->relocs().end(); ++it) {
    if (it->sym == sym && it->addend == addend && isLinxPCRelHi(it->type)) {
      after = &*it;
      break;
    }
  }