Step over syscalll insn with disp-step on and off

Step over syscalll insn with disp-step on and off

disp-step-syscall.exp was added to test displaced stepping over syscall
instructions, in which we set breakpoint on syscall instruction, and
step over it.  In fact, we can extend the test to non-displaced-stepping
case.  This patch wraps the test with displaced stepping on and off.
Note that the indentation and format isn't adjusted here to make this
patch easy to read.  The following patch will fix the format separately.

gdb/testsuite:

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

	* gdb.base/disp-step-syscall.exp: Don't invoke
	support_displaced_stepping.
	(disp_step_cross_syscall): Test with displaced stepping off and
	on if supported.