[libcxx] Disable assert when building with newlib

[tomhughes]

Summary

newlib defines blank as 0200 (0x80) (see newlib/libc/include/ctype.h),
which is the same as __regex_word, so the static_assert fails. However,
"mask" is only a single byte, so we don't have room to expand.

[tomhughes]

Updated commit message to include bug number.

[tomhughes]

I'm not sure what the implications of this static_assert failing are. Since we can't avoid overlap with newlib's current defines and mask size, do we need to get newlib to fix this upstream?

[ldionne]

I think this is being superseded by https://reviews.llvm.org/D138195. If so, please abandon to clear the review queue. Otherwise, please let us know.

[tomhughes]

Yes, it looks like https://reviews.llvm.org/D138195 addresses this.

libcxx/include/__locale

 #else
 # error unknown rune table for this platform -- do you mean to define _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE?
 #endif
     static const mask alnum  = alpha | digit;
     static const mask graph  = alnum | punct;
 
     _LIBCPP_INLINE_VISIBILITY ctype_base() {}
 
+#ifndef _NEWLIB_VERSION
     static_assert((__regex_word & ~(space | print | cntrl | upper | lower | alpha | digit | punct | xdigit | blank)) == __regex_word,
                   "__regex_word can't overlap other bits");
+#endif
 };
 
 template <class _CharT> class _LIBCPP_TEMPLATE_VIS ctype;
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <>
 class _LIBCPP_TYPE_VIS ctype<wchar_t>
     : public locale::facet,