[llvm] [ThinLTO] Properly support targets that require importing all external functions (PR #133588)

[Shilei Tian via llvm-commits]

shiltian wrote:

Functions with `available_externally` linkage are never emitted into the object file. They're eliminated later in the pipeline, which eventually leads to an undefined symbol error.

https://github.com/llvm/llvm-project/pull/133588