Small clean up of use_displaced_stepping

Small clean up of use_displaced_stepping

This function returns the result of a quite big condition.  I think it
would be more readeable if it was broken up in smaller pieces and
commented.  This is what this patch does.

I also introduced gdbarch_supports_displaced_stepping, since it shows
the intent better than checking for gdbarch_displaced_step_copy_insn_p.
I also used that new function in displaced_step_prepare_throw.

I also updated the comment on top of can_use_displaced_stepping, which
seemed a bit outdated with respect to non-stop.  The comment likely
dates from before it was possible to have targets that always operate
non-stop under the hood, even when the user-visible mode is all-stop.

No functional changes intended.

gdb/ChangeLog:

	* infrun.c (gdbarch_supports_displaced_stepping): New.
	(use_displaced_stepping): Break up conditions in smaller pieces.
	Use gdbarch_supports_displaced_stepping.
	(displaced_step_prepare_throw): Use
	gdbarch_supports_displaced_stepping.