Remove parameter sysret from linux_target_ops.get_syscall_trapinfo

When I implement linux_target_ops.get_syscall_trapinfo for aarch64 and arm,
I find the second parameter sysret isn't used at all.  In RSP, we don't
need syscall return value either, because GDB can figure out the return
value from registers content got by 'g' packet.

This patch is to remove them.

gdb/gdbserver:

2016-06-28  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (get_syscall_trapinfo): Remove parameter sysret.
	Callers updated.
	* linux-low.h (struct linux_target_ops) <get_syscall_trapinfo>:
	Remove parameter sysno.
	* linux-x86-low.c (x86_get_syscall_trapinfo): Remove parameter
	sysret.