Auto merge of #110263 - jyn514:ui-fulldeps-llvm, r=albertlarsan68

Add `libLLVM.so` to the target libdir when download-rustc is enabled

Previously, we would only add it to the host libdir, which meant it couldn't be loaded by `ui-fulldeps` tests that used rustc_private.

Fixes rust-lang/rust#110225, fixes rust-lang/rust#110226.