Include uchar.h only for bionic

Bug: http://b/111265772

uchar.h is not available in the glibc used for the host.  Since Bionic's
stdatomic.h is also used for the host (via a copy in prebuilts/clang)
guard the include with a check for __BIONIC__.

Also replace the check for C11 around definition of
atomic_char_{16,32}_t with a check for __BIONIC__.  While we're here,
check if __cplusplus is defined before checking its value.  This avoids
a potential -Wundef warning.

Test: Build
Change-Id: Icb62150d978703175b78d8ad12b42cf98b424b9a