Reorg nios2_semi_return_* to gdb_syscall_complete_cb.
Use the 32-bit version normally, and the 64-bit version
for HOSTED_LSEEK.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>