[runtimes] Fix not correctly searching target builtins directories (#103311)

Summary:
Enabling `compiler-rt` only worked previously if we had it in the
default target. This is because we didn't extract the path carefully.
This patch fixes that by getting the correct path and appending it,
instead of just the project name.

Author: jhuber6

llvm/runtimes/CMakeLists.txt

@@ -19,7 +19,12 @@ endforeach()
 function(get_compiler_rt_path path)
   set(all_runtimes ${runtimes})
   foreach(name ${LLVM_RUNTIME_TARGETS})
-    list(APPEND all_runtimes ${RUNTIMES_${name}_LLVM_ENABLE_RUNTIMES})
+    foreach(proj ${RUNTIMES_${name}_LLVM_ENABLE_RUNTIMES})
+      set(proj_dir "${CMAKE_CURRENT_SOURCE_DIR}/../../${proj}")
+      if(IS_DIRECTORY ${proj_dir} AND EXISTS ${proj_dir}/CMakeLists.txt)
+        list(APPEND all_runtimes ${proj_dir})
+      endif()
+    endforeach()
   endforeach()
   list(REMOVE_DUPLICATES all_runtimes)
   foreach(entry ${all_runtimes})