[ObjCopy] Fix type mismatch in writeCodeSignatureData()

The result of pointer subtraction is of type ptrdiff_t, which is not necessarily the same underlying type as ssize_t. This can lead to a compilation error since std::min requires both parameters to be the same type.

Fixes: https://github.com/llvm/llvm-project/issues/54846

Reviewed By: alexander-shaposhnikov, drodriguez, jhenderson

Differential Revision: https://reviews.llvm.org/D128117