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".

From-SVN: r221362

gcc/ChangeLog

+2015-03-11  Thomas Schwinge  <thomas@codesourcery.com>
+
+	* config/nvptx/nvptx.h (LIBSTDCXX): Define to "gcc".
+
 2015-03-11  Marek Polacek  <polacek@redhat.com>
 
 	PR tree-optimization/65388

gcc/config/nvptx/nvptx.h

@@ -366,4 +366,8 @@ struct GTY(()) machine_function
 #define FUNCTION_MODE QImode
 #define HAS_INIT_SECTION 1
 
+/* The C++ front end insists to link against libstdc++ -- which we don't build.
+   Tell it to instead link against the innocuous libgcc.  */
+#define LIBSTDCXX "gcc"
+
 #endif /* GCC_NVPTX_H */