libstdc++: Fix testsuite for skipping gdb tests on remote/non-native target

[Marc Poulhies]

This fixes an incorrect invocation of gdb on remote targets where DejaGNU would try to run host's gdb in remote target simulator.
gdb-test skips the testing when target is remote or non native but the gdb version check function does not.

libstdc++-v3/ChangeLog:
        * testsuite/lib/gdb-test.exp (gdb_batch_check): Exit if non native or remote target.

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