libstdc++: Enable _GLIBCXX_ASSERTIONS by default for -O0 [PR112808]

Too many users don't know about -D_GLIBCXX_ASSERTIONS and so are missing
valuable checks for C++ standard library preconditions. This change
enables libstdc++ assertions by default when compiling with -O0 so that
we diagnose more bugs by default.

When users enable optimization we don't add the assertions by default
(because they have non-zero overhead) so they still need to enable them
manually.

For users who really don't want the assertions even in unoptimized
builds, defining _GLIBCXX_NO_ASSERTIONS will prevent them from being
enabled automatically.

libstdc++-v3/ChangeLog:

	PR libstdc++/112808
	* doc/xml/manual/using.xml (_GLIBCXX_ASSERTIONS): Document
	implicit definition for -O0 compilation.
	(_GLIBCXX_NO_ASSERTIONS): Document.
	* doc/html/manual/using_macros.html: Regenerate.
	* include/bits/c++config [!__OPTIMIZE__] (_GLIBCXX_ASSERTIONS):
	Define for unoptimized builds.