[libcxx] [test] Fix max_size.pass.cpp for other Windows architectures

All current Windows architectures (i386, x86_64, arm, arm64) get
the full_size() behaviour here. x86_64 (the only one tested in CI
currently) is handled by the first ifdef at the top, but handle
Windows in general on all other architectures later.

Differential Revision: https://reviews.llvm.org/D124989