[libc] Remove global constructor in `getopt` implementation

This file required a global constructor due to copying the file stream
and have a non-constexpr constructor for the wrapper type. Also, I
changes the `opterr` to be a pointer, because it seemed like it wasn't
being set correctly as an externally visibile variable if we just
captured it by value.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D155766