[Morello] Fix displaced stepping LSB adjustment and add debugging output

[Morello] Fix displaced stepping LSB adjustment and add debugging output

This patch fixes cases where the LSB of a pure cap function isn't cleared when
we use it for address arithmetic. Also adds debugging output.

gdb/ChangeLog:

2020-10-20  Luis Machado  <luis.machado@arm.com>

	* aarch64-tdep.c (aarch64_displaced_step_data) <gdbarch>: New field.
	(aarch64_displaced_step_b): Add debugging output.
	(aarch64_displaced_step_copy_insn): Initialize the gdbarch member.
	(aarch64_pointer_to_address, aarch64_address_to_pointer)
	(aarch64_integer_to_address): Add debugging output.
	* arch/aarch64-insn.c (aarch64_decode_b, aarch64_decode_bcond)
	(aarch64_decode_cb, aarch64_decode_tb)
	(aarch64_decode_ldr_literal): Refactor and add debugging output.
	* infrun.c (displaced_step_prepare_throw): Clear LSB bits.