Rename disp-step-syscall.exp to step-over-syscall.exp

Rename disp-step-syscall.exp to step-over-syscall.exp

disp-step-syscall.exp is extended for stepping over syscall instruction
in different cases, with or without displaced stepping, and stepping
over by GDBserver.

This patch rename disp-step-syscall.exp to step-over-syscall.exp to
reflect this.

gdb/testsuite:

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

	* gdb.base/disp-step-fork.c: Rename to ...
	* gdb.base/step-over-fork.c: ... it.  New file.
	* gdb.base/disp-step-vfork.c: Rename to ...
	* gdb.base/step-over-vfork.c: ... it.  New file.
	* gdb.base/disp-step-syscall.exp: Rename to ...
	* gdb.base/step-over-syscall.exp: ... it.  New file.
	(disp_step_cross_syscall): Rename to ...
	(step_over_syscall): ... it.