It wasn't quite as nasty as I thought, but splitting it up helps add a
little self documentation and, possibly more importantly, simplify
navigation through the type checks and argument setup.
I've long felt build_function_call was getting a bit big, and expr.c
especially so. This should make it easier to rewrite build_function_call
for dealing with target-specific code. As a bonus, the int through ...
warning is already cleaned up.
