[libcxx] Fix build bustage with threads disabled

Building with -DLIBCXX_ENABLE_THREADS=OFF -DLIBCXXABI_ENABLE_THREADS=OFF
(like e.g. for wasm) fails after D146228 because of a misplaced std
namespace begin/end.

Reviewed By: philnik, #libc

Differential Revision: https://reviews.llvm.org/D146682