No libstdc++ for nvptx.

No libstdc++ for nvptx.

The C++ front end insists to link against libstdc++ -- which we don't build:

    $ < build-gcc/gcc/testsuite/g++/g++.log grep -o 'error opening [^[:cntrl:]]*' | sort | uniq -c
          2 error opening libasan.a
          2 error opening libssp.a
      12075 error opening libstdc++.a

Based on GCC trunk r220892:

                    === g++ Summary ===

    # of expected passes            [-63221-]{+68841+}
    # of unexpected failures        [-11751-]{+8764+}
    # of unexpected successes       6
    # of expected failures          [-246-]{+249+}
    # of unresolved testcases       [-5950-]{+3353+}
    # of unsupported tests          [-4160-]{+4143+}

	gcc/
	* config/nvptx/nvptx.h (LIBSTDCXX): Define to "gcc".

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@221362 138bc75d-0d04-0410-961f-82ee72b054a4