The `bool serial` condition in scanRelocations disabled parallelism for
three cases: -z nocombreloc, MIPS, and PPC64. Resolve two cases:
- nocombreloc: .rela.dyn is now always created with combreloc=true so
non-relative relocations are sorted deterministically. Since
#187964 already separates relative relocations unconditionally,
the only remaining effect of -z nocombreloc is suppressing
DT_RELACOUNT (gated on ctx.arg.zCombreloc in DynamicSection).
- PPC64: After #181496 moved scanning into scanSectionImpl, the
sole thread-unsafe access is ctx.ppc64noTocRelax (DenseSet::insert).
Protect it with ctx.relocMutex, which is already used for rare
operations during parallel scanning.
MIPS retains serial scanning due to `MipsGotSection` mutations.
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