[llvm] [Offload] Make MemoryManager threshold ENV var size_t type. (PR #124063)

[Shilei Tian via llvm-commits]

shiltian wrote:

`size_t` is more for platform portable. We decided to not support 32-bit at some point in the past so using `uint64_t` is fine.

https://github.com/llvm/llvm-project/pull/124063