[Morello] Generate target descriptions based on runtime capability feature checks

[Morello] Generate target descriptions based on runtime capability feature checks

This patch adds code to do runtime checks for Morello, so GDB can pick the
correct target description and register set.

gdb/ChangeLog:

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

	* aarch64-linux-nat.c (aarch64_linux_nat_target::read_description):
	Check for HWCAP2_MORELLO.
	* aarch64-linux-tdep.c (aarch64_linux_core_read_description): Likewise.
	* aarch64-tdep.c (tdesc_aarch64_list): Add one more dimension.
	(aarch64_read_description): New parameter capability_p, use it to
	generate the proper target description.
	(aarch64_gdbarch_init): Update invocation of aarch64_read_description.
	* aarch64-tdep.h (aarch64_read_description): New parameter capability_p.
	* arch/aarch64.c (aarch64_create_target_description): New parameter
	capability_p. Use it.
	* arch/aarch64.h (aarch64_create_target_description): New parameter
	capability_p.

gdbserver/ChangeLog:

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

	* linux-aarch64-ipa.cc (get_ipa_tdesc): Update.
	* linux-aarch64-low.cc (aarch64_target::low_arch_setup): Check for
	HWCAP2_MORELLO and use it.
	* linux-aarch64-tdesc.cc (tdesc_aarch64_list): Add one more dimension.
	(aarch64_linux_read_description): New parameter capability_p. Use it.
	* linux-aarch64-tdesc.h (aarch64_linux_read_description): New parameter
	capability_p.