[LLVM] Add per-target runtime directory to rpath (llvm#199755)

Summary:
    This simply adds the LLVM_DEFAULT_TARGET_TRIPLE to the LLVM build's
rpath if present. This keeps things hermetic for the library (offload)
    that depends on it.
    
  The reason this is required is because `llvm-gpu-loader` calls
`DynamicLibrary` on the Offload runtime. However, in a shared library
build the actual call is in libLLVMSupport.so, which does not have this
RPath, so `dlopen` delegates to that which does not know how to find it.
The only options to fix this are to use `dlopen` directly in the loader,
    or add the rpath to the LLVM binaries.
    
I think this makes sense for LLVM, because the target-specific directory
    can contain LLVM related libraries.

Author: jhuber6

llvm/cmake/modules/AddLLVM.cmake

@@ -2691,6 +2691,10 @@ function(llvm_setup_rpath name)
   elseif(UNIX)
     set(_build_rpath "\$ORIGIN/../lib${LLVM_LIBDIR_SUFFIX}" ${extra_libdir})
     set(_install_rpath "\$ORIGIN/../lib${LLVM_LIBDIR_SUFFIX}")
+    if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
+      list(APPEND _build_rpath "\$ORIGIN/../lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE}")
+      list(APPEND _install_rpath "\$ORIGIN/../lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE}")
+    endif()
     if("${CMAKE_SYSTEM_NAME}" MATCHES "(FreeBSD|DragonFly)")
       set_property(TARGET ${name} APPEND_STRING PROPERTY
                    LINK_FLAGS " -Wl,-z,origin ")