Define atomic_charN_t only if charN_t is supported.

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 8b002362)
Change-Id: Ie5a17f20b8b545c97128d00605b4eabd2a6bfe3e