[CMake] Install LLVMgold.so for LLVM_INSTALL_TOOLCHAIN_ONLY=on (#87567)

LLVMgold.so can be used with GNU ar, gold, ld, and nm to process LLVM
bitcode files. Install it in LLVM_INSTALL_TOOLCHAIN_ONLY=on builds like
we install libLTO.so.

Suggested by @emelife

Fix #84271