gdb: add target_ops::supports_displaced_step

A previous patch added some `displaced_step_prepare` and
`displaced_step_finish` methods to allow a target_ops to implement
displaced stepping.  The default implementation is to defer to the
gdbarch.

However, there is still a `gdbarch_supports_displaced_stepping` check in
infrun.c, that decides if we should use displaced stepping or not.  So
if a target_ops implemented displaced stepping, independently of the
gdbarch, displaced stepping wouldn't be used.

Add the target_ops::supports_displaced_step method, with a default
behavior of checking if the gdbarch supports it (which is in sync with
the default implementations of displaced_step_prepare and
displaced_step_finish).