Auto merge of #31283 - dotdash:32_64_cc, r=alexcrichton

Auto merge of #31283 - dotdash:32_64_cc, r=alexcrichton

When cross compiling for a target that has a larger usize type than the
host system, we use a truncated value to mark data as dropped,
eventually leading to drop calls on already dropped data. To properly
handle this, the drop pattern needs to be of type u64.

Since C_integral truncates its given value to the requested size anyway,
we can also drop the function that chose between the u32 and u64 values,
and always use the u64 constant.

Fixes #31139

r? @pnkfelix