[ELF] Optimize binary search in getSectionPiece (#187916)
Two optimizations to make getSectionPiece O(1) for common cases:
1. For non-string fixed-size merge sections, use direct computation
(offset / entsize) instead of binary search.
2. Pre-resolve piece indices for non-section Defined symbols during
splitSections. The piece index and intra-piece offset are packed
into Defined::value as ((pieceIdx+1) << 32) | intraPieceOffset,
replacing repeated binary searches (MarkLive, includeInSymtab,
getRelocTargetVA) with a single upfront resolution.
On x86-64, references to mergeable strings use local labels:
leaq .LC0(%rip), %rax # R_X86_64_PC32 .LC0-4
The relocations use non-section symbols and benefit from optimization 2.
On many other targets (e.g. AArch64), the addend is 0 and the assembler
adjusts such relocations to reference section symbols, which still use
binary search.
On a clang link (clang-relassert reproduce tarball, x86-64):
- --gc-sections: 1.05x as fast
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