libstdc++: Move std::error_category symbol to separate file [PR117630]

As described in PR 117630 the cow-stdexcept.cc file pulls in symbols
from system_error.cc, which are not actually needed there. Moving the
definition of error_category::_M_message to a separate file should solve
it.

libstdc++-v3/ChangeLog:

	PR libstdc++/117630
	* src/c++11/Makefile.am: Add new file.
	* src/c++11/Makefile.in: Regnerate.
	* src/c++11/cow-stdexcept.cc (error_category::_M_message): Move
	member function definition to ...
	* src/c++11/cow-system_error.cc: New file.