[CMake] Install LLVMgold.so for LLVM_INSTALL_TOOLCHAIN_ONLY=on

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 llvm#84271

llvm/tools/gold/CMakeLists.txt

@@ -12,7 +12,7 @@ if( LLVM_ENABLE_PIC AND LLVM_BINUTILS_INCDIR )
      TargetParser
      )
 
-  add_llvm_library(LLVMgold MODULE
+  add_llvm_library(LLVMgold MODULE INSTALL_WITH_TOOLCHAIN
     gold-plugin.cpp
     )