Re-enable LTCG optimization for vendored libraries

[jkotas]

          Yes, our current build setup isn't conducive to building two flavors of our vendored libraries (we would need to be able to define separate target names and include the subdirectories twice).

There are alternative ways we could include the vendored libs, but that would require significant changes to how we build. I think that's something we can consider in .NET 10 but in a later PR (especially if we want this in P1).

Originally posted by @jkoritzinsky in #111805 (comment)

[jkotas]

cc @jkoritzinsky

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