[lld-macho] Use uint64_t for getSize() instead of size_t

Summary:
So things work on 32-bit machines. (@vzakhari reported the
breakage starting from D80177).

Reviewers: #lld-macho, vzakhari

Subscribers: llvm-commits, vzakhari

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81982