Plus a function call (likely with GOT indirection) plus another pointer dereference to get from the locale handle to the array. Also you now need some initialization code to create the locale and store the handle somewhere (unless you want to make this even more ridiculously inefficient and create it each time).

Also, five of those whitespace characters have contiguous byte values so you don't even need 6 comparisons. The compiler doesn't have to keep the short circuting here.