[Morello] Add set/show ABI command for AArch64

[Morello] Add set/show ABI command for AArch64

Add a new command for developers to set and show the AArch64 ABI GDB is
using at the moment.

We define 2 ABI's: AAPCS64 and AAPCS64-cap.

Each of these ABI's should impact the architecture setup in different ways.

gdb/ChangeLog:

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

	* aarch64-tdep.c (set_aarch64_cmdlist, show_aarch64_cmdlist,
	aarch64_abi_strings, aarch64_current_abi_global)
	(aarch64_current_abi_string): New static globals.
	(aarch64_update_current_architecture, aarch64_set_abi)
	(aarch64_show_abi): New functions.
	(aarch64_gdbarch_init): Handle ABI identification.
	(_initialize_aarch64_tdep): Add new ABI commands.
	* aarch64-tdep.h (aarch64_abi_kind): New enum.
	(struct gdbarch_tdep) <abi>: New field.