gdb: don't require displaced step copy_insn to be implemented in prepare/finish are

gdb: don't require displaced step copy_insn to be implemented in prepare/finish are

The gdbarch verification currently requires that the gdbarch
displaced_step_prepare and displaced_step_finish methods are provided if
displaced_step_copy_insn is (and not provided if it isn't).

displaced_step_copy_insn is used by the
multiple_displaced_buffer_manager class, but a gdbarch could very well
decide to implement prepare and finish without that, and without
requiring copy_insn.

Remove the dependency in the gdbarch verification between prepare/finish
and copy_insn.  Now, prepare and finish must be both provided or both
not-provided.

Since multiple_displaced_buffer_manager uses the `copy_insn` and `fixup`
gdbarch methods, it now asserts that they are present for the thread
architecture in its `prepare` method.