This patch updates the nios2-linux code for the new Linux generic system
call interface. Mainly removing (non-rt) sigreturn related definitions,
and updating the rt_sigreturn syscall NR.

Will commit after Sandra's approval.

Thanks,
Chung-Lin

2013-12-01  Chung-Lin Tang  <cltang@codesourcery.com>

        gdb/
        * nios2-linux-tdep.c (nios2_linux_sigreturn_init): Remove.
        (nios2_linux_sigreturn_tramp_frame): Remove.
        (nios2_linux_rt_sigreturn_tramp_frame): Update rt_sigreturn syscall
        number.
        (nios2_linux_syscall_next_pc): Likewise. Remove sigreturn case.
        (nios2_linux_init_abi):
        Remove registration of nios2_linux_sigreturn_tramp_frame.