ARM: reuse existing libcall global variable if possible.

If we try to create a new GlobalVariable on each iteration, the Module will
detect the name collision and "helpfully" rename later iterations by appending
".1" etc. But "___udivsi3.1" doesn't exist and we definitely don't want to try
to call it.

So instead check whether there's already a global with the right name in the
module and use that if so.