[amdgcn] Silence warnings + add gcc_unreachable()

[amdgcn] Silence warnings + add gcc_unreachable()

        * config/gcn/gcn.c (gcn_expand_scalar_to_vector_address,
        gcn_md_reorg): Remove unused variables.
        (gcn_emutls_var_init): Add missing (but unreachable) return
        to silence warning.
        (gcn_hsa_declare_function_name): Add gcc_unreachable to ensure
        target != TARGET_GCN3 or TARGET_GCN3 will fail instead of use
        an uninitialized variable.

From-SVN: r278678