[libc++] Add a libc++ configuration that does not support localization

When porting libc++ to embedded systems, it can be useful to drop support
for localization, which these systems don't implement or care about.

Differential Revision: https://reviews.llvm.org/D90072

libcxx/test/std/strings/basic.string/string.nonmembers/string.io/lit.local.cfg

@@ -0,0 +1,3 @@
+# These std::string functions require iostreams, which requires localization
+if 'libcpp-has-no-localization' in config.available_features:
+  config.unsupported = True