libstdc++: Downgrade DEBUG to ASSERTIONS when !HOSTED

Supporting the debug mode in freestanding is a non-trivial job, so
instead, as a best-effort, enable assertions, which are light and easy.

libstdc++-v3/ChangeLog:

	* include/bits/c++config: When __STDC_HOSTED__ is zero,
	disable _GLIBCXX_DEBUG and, if it was set, enable
	_GLIBCXX_ASSERTIONS.
	* testsuite/lib/libstdc++.exp (check_v3_target_debug_mode):
	Include <bits/c++config.h> when determining whether debug is
	set, in order to inherit the logic from above

libstdc++-v3/include/bits/c++config

@@ -397,6 +397,13 @@ _GLIBCXX_END_NAMESPACE_VERSION
 # define _GLIBCXX_END_INLINE_ABI_NAMESPACE(X)   } // inline namespace X
 #endif
 
+// In the case that we don't have a hosted environment, we can't provide the
+// debugging mode.  Instead, we do our best and downgrade to assertions.
+#if defined(_GLIBCXX_DEBUG) && !__STDC_HOSTED__
+#undef _GLIBCXX_DEBUG
+#define _GLIBCXX_ASSERTIONS 1
+#endif
+
 // Inline namespaces for special modes: debug, parallel.
 #if defined(_GLIBCXX_DEBUG) || defined(_GLIBCXX_PARALLEL)
 namespace std

libstdc++-v3/testsuite/lib/libstdc++.exp

@@ -1007,6 +1007,7 @@ proc check_v3_target_debug_mode { } {
     global cxxflags
     return [check_v3_target_prop_cached et_debug_mode {
 	set code "
+	#include <bits/c++config.h>
 	#if ! defined _GLIBCXX_DEBUG
 	# error no debug mode
 	#endif