D: Fix name collision between im D module function pointer and actual C function.

Previously, the function pointers were not only declared with
extern(C) calling convention, but actually had C linkage
themselves. Thus, they were exported under their bare names,
potentially colliding with the actual function definitions
in the wrapped library if the dynamic linker decided to
resolve them the wrong way.

This fixes the sneaky1 test case, although I have no idea why
the add() reference in D_add() (via the PLT) is rebound to the
function pointer there and not in all other test cases and
real-world libraries. As far as I can see, there don't seem to
be any special symbol visibility/binding settings involved in
our build system.