The name __null_sentinel is defined as a macro by newlib, so we can't
use it as an identifier. That variable is not actually used by
libstdc++, it was added because P2728R6 proposes std::uc::null_sentinel.
Since we don't need it and it breaks bootstrap for newlib targets, just
remove it. A null sentinel can still be used by constructing a
_Null_sentinel_t object as needed, rather than having a named object of
that type predefined.
libstdc++-v3/ChangeLog:
* include/bits/unicode.h (__null_sentinel): Remove.
* testsuite/17_intro/names.cc: Add __null_sentinel.
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