libstdc++: Stop defining C++0x compat symbols for versioned namespace

libstdc++: Stop defining C++0x compat symbols for versioned namespace

The src/c++11/compatibility*-c++0x.cc files define symbols that need to
be exported for ancient versions of libstdc++.so.6 due to changes
between C++0x and the final C++11 standard. Those symbols are not needed
in the libstdc++.so.8 library, and we can skip building them entirely.

This also fixes the build failure I introduced last week when making the
versioned namespace config not use the _V2 namespace for compat symbols.

libstdc++-v3/ChangeLog:

	* src/Makefile.am [ENABLE_SYMVERS_GNU_NAMESPACE] (cxx11_sources):
	Do not build the compatibility*-c++0x.cc objects.
	* src/Makefile.in: Regenerate.
	* src/c++11/compatibility-c++0x.cc [_GLIBCXX_INLINE_VERSION]:
	Refuse to build for the versioned namespace.
	* src/c++11/compatibility-chrono.cc: Likewise.
	* src/c++11/compatibility-condvar.cc: Likewise.
	* src/c++11/compatibility-thread-c++0x.cc: Likewise.
	* src/c++11/chrono.cc (system_clock, steady_clock):
	Use macros to define in inline namespace _V2, matching the
	declarations in <system_error>.
	* src/c++11/system_error.cc (system_category, generic_category):
	Likewise.

(cherry picked from commit 357d6fcd41e771128226b7916166f537b2d53a29)