AArch64: Add pointer authentication feature

AArch64: Add pointer authentication feature

Pointer Authentication is a new feature in AArch64 v8.3-a. When enabled in
the compiler, function return addresses will be mangled by the kernel.

Add register description xml and wire up to aarch64_linux_read_description.
This description includes the two pauth user registers.

Nothing yet uses the feature - that is added in later patches.

gdb/ChangeLog:

	* aarch64-linux-nat.c
	(aarch64_linux_nat_target::read_description): Add pauth param.
	* aarch64-linux-tdep.c
	(aarch64_linux_core_read_description): Likewise.
	* aarch64-tdep.c (struct target_desc): Add in pauth.
	(aarch64_read_description): Add pauth param.
	(aarch64_gdbarch_init): Likewise.
	* aarch64-tdep.h (aarch64_read_description): Likewise.
	* arch/aarch64.c (aarch64_create_target_description): Likewise.
	* arch/aarch64.h (aarch64_create_target_description): Likewise.
	* features/Makefile: Add new files.
	* features/aarch64-pauth.c: New file.
	* features/aarch64-pauth.xml: New file.

gdb/doc/ChangeLog:

	* gdb.texinfo: Describe pauth feature.

gdb/gdbserver/ChangeLog:

	* linux-aarch64-ipa.c (get_ipa_tdesc): Add pauth param.
	(initialize_low_tracepoint): Likewise.
	* linux-aarch64-low.c (aarch64_arch_setup): Likewise.
	* linux-aarch64-tdesc-selftest.c (aarch64_tdesc_test): Likewise.
	* linux-aarch64-tdesc.c (struct target_desc): Likewise.
	(aarch64_linux_read_description): Likewise.
	* linux-aarch64-tdesc.h (aarch64_linux_read_description): Likewise.