[lld] [lld][macho] Restructure thunk generation algorithm (PR #193367)

[Kyungwoo Lee via llvm-commits]

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kyulee-com wrote:

The code was already there, but `needsThunks()` appears assuming `__text` precedes `__stubs` and `__objc_stubs` in the `__TEXT` segment in order. Can we make a simple comment on the top?

https://github.com/llvm/llvm-project/pull/193367