[libc] Fix '/gpu' directory not being made for the declarations

Summary:
We use this extra directory for offloading languages like CUDA or
OpenMP. We made the '/gpu' directory for the regular headers but not the
others. Fix that for now.

libc/cmake/modules/LLVMLibCHeaderRules.cmake

@@ -141,6 +141,7 @@ function(add_gen_header target_name)
 
   if(LIBC_TARGET_OS_IS_GPU)
     file(MAKE_DIRECTORY ${LIBC_INCLUDE_DIR}/llvm-libc-decls)
+    file(MAKE_DIRECTORY ${LIBC_INCLUDE_DIR}/llvm-libc-decls/gpu)
     set(decl_out_file ${LIBC_INCLUDE_DIR}/llvm-libc-decls/${relative_path})
     add_custom_command(
       OUTPUT ${decl_out_file}