Guard include of uchar.h in stdatomic.h

Bionic stdatomic.h is also used for the host (via a copy in
prebuilts/clang).  Revert to guarding the include of uchar.h based on
__STDC_VERSION__, so it is included only when needed.

Change-Id: I5b45c7f5d16da223478512104702a3e4e5a975ae
Test: bionic tests on host and Angler.  Confirmed failures I am 
      seeing are KIs.