libstdc++: Downgrade DEBUG to ASSERTIONS when !HOSTED

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