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

[via llvm-commits]

hidekisaito wrote:

In the interest of moving this quicker, I changed it to use UInt64Envar instead. I can try create/use SizeTEnvar after this patch goes in.

@jhuber6, thanks for the approval. When the test passes, please merge.

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