Some platform code is apparently compiled with switches that do
not support char16_t and char32_t. This caused stdatomic.h to fail
to compile. This CL makes stdatomic.h usable in those environments.
(cherry picked from commit 8b002362d996859ebfc8588d6859a9a79203dc27)
Change-Id: Ie5a17f20b8b545c97128d00605b4eabd2a6bfe3e