Extend step-over-syscall.exp with different detach-on-fork and follow-fork modes

Extend step-over-syscall.exp with different detach-on-fork and follow-fork modes

This patch extends step-over-syscall.exp by setting different values to
detach-on-fork and follow-fork.

gdb/testsuite:

2016-06-17  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/step-over-syscall.exp (break_cond_on_syscall): New
	parameters follow_fork and detach_on_fork.  Set follow-fork-mode
	and detach-on-fork.  Adjust tests.
	(top level): Invoke break_cond_on_syscall with combinations of
	syscall, follow-fork-mode and detach-on-fork.