Make it possible to link against libstdc++ as well as libsupc++ with CMake.

Linking against libstdc++, rather than libsupc++, is probably better
for people who need to link against clients of libstdc++.  Because
libsupc++ is provided only as a static library, its globals are not
shared between the static library and the copy linked into libstdc++.
This has been found to cause at least one test failure.

This also removes a number of symbols which were multiply defined
between libstdc++ and libc++, only when linking with libstdc++.

Differential Revision: http://llvm-reviews.chandlerc.com/D1825

llvm-svn: 192075