Fixed uchar.h for clang3.6

The NDK self-test issue64679-prctl failed to build because by default
clang3.6 builds in C11 mode, but the ifdef in the file 'uchar.h' hid the
required char16_t and char32_t declarations based on the GCC version,
which is declared as 4 by clang.

The ifdef is changed so that it is based on C version rather than a
compiler version. This way it is compatible with gcc 4, gcc 5 and clang
which are actually the only used toolchains.

See also: b.android.com/177609

Change-Id: I78c4bdc681c6f8a31b92dbe8ffe35e84ef2eda36
Signed-off-by: Egor Kochetov <egor.kochetov@intel.com>