Fix the type of offset that broke 32-bit flang-rt build to use uint64_t consistently

[DanielCChen]

The recent change of flang-rt has code like std::size_t offset{offset_};.
It broke the 32-bit flang-rt build because Component::offset_ is of type uint64_t but size_t varies.
Clang complains

error: non-constant-expression cannot be narrowed from type 'std::uint64_t' (aka 'unsigned long long') to 'std::size_t' (aka 'unsigned long') in initializer list [-Wc++11-narrowing]
  143 |   std::size_t offset{offset_};
      |                      ^~~~~~~

This patch is to use the consistent uint64_t for offset.

[klausler]

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

[DanielCChen]

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

Yeah. That would work too. Will update.