[OpenMP] Fix 'libc' configuration when building OpenMP

Summary:
Forgot to port this option's old handling from offload. It's not way
easier since they're built in the same CMake project. Also delete the
leftover directory that's not used anymore, don't know how that was
still there.

Author: jhuber6

offload/DeviceRTL/CMakeLists.txt

@@ -64,7 +64,7 @@ set_target_properties(libompdevice PROPERTIES
   RUNTIME_OUTPUT_NAME libomptarget-${target_name}.bc)
 
 # If the user built with the GPU C library enabled we will use that instead.
-if(LIBOMPTARGET_GPU_LIBC_SUPPORT)
+if(TARGET libc)
   target_compile_definitions(libompdevice PRIVATE OMPTARGET_HAS_LIBC)
 endif()
 target_compile_definitions(libompdevice PRIVATE SHARED_SCRATCHPAD_SIZE=512)