[libc] Skip tests that depend on undefined targets

[jhuber6]

Summary

When we search through the dependencies for a test we somtimes query
get_target_property on a target that has not been defined. This is the
case for targets that have minimal entrypoints defined such as the WIP
GPU target. This patch changes all tests whose dependencies are
undefined to be skipped entirely.

libc/cmake/modules/LLVMLibCTestRules.cmake

 #   If skipped entrypoints were found, then <skipped_entrypoints_var> is
 #   set to a true value.
 #   targetN is either an "add_entrypoint_target" target or an
 #   "add_object_library" target.
 function(get_object_files_for_test result skipped_entrypoints_list)
   set(object_files "")
   set(skipped_list "")
   foreach(dep IN LISTS ARGN)
+    if (NOT TARGET ${dep})
+      # Skip any tests whose dependencies have not been defined.
+      list(APPEND skipped_list ${dep})
+      continue()
+    endif()
     get_target_property(dep_type ${dep} "TARGET_TYPE")
     if(NOT dep_type)
       # Target for which TARGET_TYPE property is not set do not
       # provide any object files.
       continue()
     endif()
 
     if(${dep_type} STREQUAL ${OBJECT_LIBRARY_TARGET_TYPE})