[AArch64] Make gdbserver register set selection dynamic

[AArch64] Make gdbserver register set selection dynamic

The current register set selection mechanism for AArch64 is static, based
on a pre-populated array of register sets.

This means that we might potentially probe register sets that are not
available. This is OK if the kernel errors out during ptrace, but probing the
tag_ctl register, for example, does not result in a ptrace error if the kernel
supports the tagged address ABI but not MTE (PR 28355).

Making the register set selection dynamic, based on feature checks, solves
this and simplifies the code a bit. It allows us to list all of the register
sets only once, and pick and choose based on HWCAP/HWCAP2 or other properties.

gdb/ChangeLog:

2021-11-03  Luis Machado  <luis.machado@linaro.org>

	PR gdb/28355

	* arch/aarch64.h (struct aarch64_features): New struct.

gdbserver/ChangeLog:

2021-11-03  Luis Machado  <luis.machado@linaro.org>

	PR gdb/28355

	* linux-aarch64-low.cc (is_sve_tdesc): Remove.
	(aarch64_target::low_arch_setup): Rework to adjust the register sets.
	(aarch64_regsets): Update to list all register sets.
	(aarch64_regsets_info, regs_info_aarch64): Replace NULL with nullptr.
	(aarch64_sve_regsets, aarch64_sve_regsets_info)
	(regs_info_aarch64_sve): Remove.
	(aarch64_adjust_register_sets): New.
	(aarch64_target::get_regs_info): Remove references to removed structs.
	(initialize_low_arch): Likewise.