libstdc++: Move __glibcxx_assert_fail to its own file

libstdc++: Move __glibcxx_assert_fail to its own file

This avoids a dependency on the other symbols in src/c++11/debug.o when
linking statically to libstdc++.a without using -Wl,--gc-sections.

libstdc++-v3/ChangeLog:

	* src/c++11/Makefile.am: Add new file.
	* src/c++11/Makefile.in: Regenerate.
	* src/c++11/debug.cc (__glibcxx_assert_fail): Move to ...
	* src/c++11/assert_fail.cc: New file.

(cherry picked from commit c7db9000fa7caceadb4e72dcc6226abebf7a6239)