Refactor gdb.base/disp-step-syscall.exp for general step over test

Refactor gdb.base/disp-step-syscall.exp for general step over test

This patch moves some code out of disp_step_cross_syscall to a new proc
check_pc_after_cross_syscall and setup.  Procedure setup is to start a
fresh GDB and compute the syscall instruction address.

gdb/testsuite:

2016-03-03  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/disp-step-syscall.exp (check_pc_after_cross_syscall): New
	proc.
	(setup): New proc.
	(disp_step_cross_syscall): Move code to check_pc_after_cross_syscall
	and setup.