[llvm] Fix the type of offset that broke 32-bit flang-rt build to use `uint64_t` consistently (PR #147359)

[Daniel Chen via llvm-commits]

DanielCChen wrote:

> Why not use a `static_cast<std::size_t>(...)` instead?

Yeah. That would work too. Will update.

https://github.com/llvm/llvm-project/pull/147359