This patch teach GDB to return the new ARMv8.3-A Pointer Authentication feature
description when there is related support.

   * A new "pauth" feature description file is added.
     The new pointer authentication feature, "pauth", contains two virtual
     registers, "pauth_dmask" and "pauth_cmask".  Their values are supplied by
     kernel to GDB through ptrace interface.  They are masks to indicate which
     bits will be signature when data pointer or code pointer is signed.  Those
     bits will be set to "1" in the mask.

   * HWCAP is used to detect whether the hardware support exists.
     Currently we only support return address signing with A key, so using ptrace
     trial read would not be reliable as it will return successfully when other
     keys are used inside kernel.

The change on include/elf/common.h needs approval from Binutils maintainers.

gdb/
2017-08-09  Jiong Wang<jiong.wang@arm.com>
             Yao Qi<yao.qi@linaro.org>

	* aarch64-linux-nat.c (fetch_pauth_masks_from_thread): New
	function.
	(aarch64_linux_fetch_inferior_registers): Support pauth
	description.
	(aarch64_linux_read_description): Likewise.
	* aarch64-tdep.c: #include "features/aarch64-pauth.c".
	(aarch64_pauth_register_names): New register names array for
	pauth.
	(aarch64_cannot_store_register): New function.
	(aarch64_tdep_has_pauth_p): New function.
	(aarch64_gdbarch_init): Handle pauth description.  Initialize
	the optional "regnum" field.  Register cannot_store_register hook.
	(_initialize_aarch64_tdep): Call initialize_tdesc_aarch64_pauth.
	* aarch64-tdep.h (struct aarch64_optional_regnum): New structure
	type.
	(struct gdbarch_tdep) <regnum>: New field.
  	(tdesc_aarch64_pauth): New variable declaration.
	(aarch64_tdep_has_pauth_p): New function declaration.
	* nat/aarch64-linux.c: Include <sys/auxv.h>.
	(aarch64_host_hwcap): New function.
	* nat/aarch64-linux.h (HWCAP_APIA): New macro.
	(aarch64_host_hwcap): New function declaration.
	* features/Makefile (WHICH): Add aarch64-pauth.
	(XMLTOC): Add aarch64-pauth.xml.
	* features/aarch64-pauth-core.xml: New file.
	* features/aarch64-pauth.c: Generated.
	* features/aarch64-pauth.xml: New file.
	* regformats/aarch64-pauth.dat: Generated.

include/
	* elf/common.h (NT_ARM_PAC_MASK): New.

gdb/doc/
	* gdb.texinfo (AArch64 Features): New feature
	"org.gnu.gdb.aarch64.pauth".