Fix step-over-syscall.exp failure

Fix step-over-syscall.exp failure

In particular, this one:

FAIL: gdb.base/step-over-syscall.exp: fork: displaced=on: check_pc_after_cross_syscall: single step over fork final pc

When ptrace fork event reporting is enabled, GDB gets a PTRACE_EVENT_FORK
event whenever the inferior executes the fork syscall.

Then the logic is that GDB needs to step the inferior yet again in order to
receive a predetermined SIGTRAP, but no execution takes place because the
signal was already queued for delivery. That means the PC should stay the same.

I noticed the aarch64 code is currently adjusting the PC in this situation,
making the inferior skip an instruction without executing it.

The following change checks if we did not execute the instruction
(pc - to == 0), making proper adjustments for such case.

Regression tested on aarch64-linux-gnu on the tryserver.

gdb/ChangeLog:

2020-01-21  Luis Machado  <luis.machado@linaro.org>

	* aarch64-tdep.c (struct aarch64_displaced_step_closure )
	<pc_adjust>: Adjust the documentation.
	(aarch64_displaced_step_fixup): Check if PC really moved before
	adjusting it.