[llvm] [gsymutil] Fix a warning on systems with 32-bit `off_t` (PR #189524)

[Justin Bogner via llvm-commits]

bogner wrote:

> Those two lines LGTM. Feel free to merge as is. With that said, it seems there are also [a few other `off_t` in the gsym code (6 total)](https://github.com/search?q=repo%3Allvm%2Fllvm-project%20path%3Allvm%2Flib%2FDebugInfo%2FGSYM%20off_t&type=code). So I wonder if it makes more sense to update all of them at once if that's not too much trouble for you (esp. the one which is in the same function that you fixed).

This makes a ton of sense to me, I've updated the other similar `off_t` usages as well.

https://github.com/llvm/llvm-project/pull/189524