depends: add Qt -ltcg for Darwin, drop it for Windows #32496

1. 
   fanquake commented at 11:45 am on May 14, 2025: member

   The related Windows patches were dropped in 5e794e62024eef612e1fbb71c76ea54d17435c14, and “Cross-compiling does not support LTO.” (from #30997).

   Using this with native Darwin builds works ok.

2. depends: enable -ltcg in darwin qt when using LTO 4b72469a3b
3. depends: remove support for Windows Qt LTO builds

   The patches needed to support this were dropped in
   5e794e62024eef612e1fbb71c76ea54d17435c14 and "Cross-compiling does not
   support LTO." (from #30997), so remove it for now.

   815f10a105
4. 
   DrahtBot commented at 11:45 am on May 14, 2025: contributor

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   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32496.

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5. DrahtBot added the label Build system on May 14, 2025

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