[patch, nios2] Update Linux parts for new syscall ABI

[Joel Brobecker]

> 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.

This is OK for me, but I think a change of this kind requires a NEWS
entry as well, to document on which systems the new GDB will be working.

-- 
Joel