Fix issue #3802 - Fix size_t size for 32bit ABI on 64bit architectures.

[JohanEngelen]

I don't know how to trigger the _d_array_slice_copy simplification by SimplifyDRuntimeCalls, so can't test that part of the PR.

[kinke]

This triggers the IR pass (call optimized away with -O, not optimized with -O -disable-d-passes):

void foo(int[] a, int[] b)
{
    a[] = b[];
}

auto bar()
{
    int[2] a = [1, 2];
    int[2] b = [3, 4];
    foo(a, b);
    return a;
}

[kinke]

I took the liberty of extending the test, fixing the target C long size as suggested and predefining D_X32, to make this land in v1.27.1.