Add NetBSD/aarch64 gdbserver support

The support is on par with NetBSD/amd64, thus GPR works,
single step and software breakpoint are operational, and the
SVR4 r_debug integration is functional.

gdbserver/ChangeLog:

         * netbsd-aarch64-low.cc: Add.
         * Makefile.in (SFILES): Register "netbsd-aarch64-low.c".
         * configure.srv: Add aarch64*-*-netbsd*.

gdbserver/netbsd-aarch64-low.cc

@@ -0,0 +1,113 @@
+/* Copyright (C) 2020 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <sys/types.h>
+#include <sys/ptrace.h>
+#include <limits.h>
+
+#include "server.h"
+#include "netbsd-low.h"
+#include "arch/aarch64.h"
+#include "arch/aarch64-insn.h"
+#include "tdesc.h"
+
+/* The fill_function for the general-purpose register set.  */
+
+static void
+netbsd_aarch64_fill_gregset (struct regcache *regcache, char *buf)
+{
+  struct reg *r = (struct reg *) buf;
+
+#define netbsd_aarch64_collect_gp(regnum, fld) do {		\
+    collect_register (regcache, regnum, &r->fld);		\
+  } while (0)
+
+  for (size_t i = 0; i < ARRAY_SIZE (r->r_reg); i++)
+    netbsd_aarch64_collect_gp (AARCH64_X0_REGNUM + i, r_reg[i]);
+
+  netbsd_aarch64_collect_gp (AARCH64_SP_REGNUM, r_sp);
+  netbsd_aarch64_collect_gp (AARCH64_PC_REGNUM, r_pc);
+}
+
+/* The store_function for the general-purpose register set.  */
+
+static void
+netbsd_aarch64_store_gregset (struct regcache *regcache, const char *buf)
+{
+  struct reg *r = (struct reg *) buf;
+
+#define netbsd_aarch64_supply_gp(regnum, fld) do {		\
+    supply_register (regcache, regnum, &r->fld);		\
+  } while(0)
+
+  for (size_t i = 0; i < ARRAY_SIZE (r->r_reg); i++)
+    netbsd_aarch64_supply_gp (AARCH64_X0_REGNUM + i, r_reg[i]);
+
+  netbsd_aarch64_supply_gp (AARCH64_SP_REGNUM, r_sp);
+  netbsd_aarch64_supply_gp (AARCH64_PC_REGNUM, r_pc);
+}
+
+/* Description of all the aarch64-netbsd register sets.  */
+
+static const struct netbsd_regset_info netbsd_target_regsets[] =
+{
+  /* General Purpose Registers.  */
+  {PT_GETREGS, PT_SETREGS, sizeof (struct reg),
+  netbsd_aarch64_fill_gregset, netbsd_aarch64_store_gregset},
+  /* End of list marker.  */
+  {0, 0, -1, NULL, NULL }
+};
+
+/* NetBSD target op definitions for the aarch64 architecture.  */
+
+class netbsd_aarch64_target : public netbsd_process_target
+{
+protected:
+  const netbsd_regset_info *get_regs_info () override;
+
+  void low_arch_setup () override;
+};
+
+/* Return the information to access registers.  */
+
+const netbsd_regset_info *
+netbsd_aarch64_target::get_regs_info ()
+{
+  return netbsd_target_regsets;
+}
+
+/* Architecture-specific setup for the current process.  */
+
+void
+netbsd_aarch64_target::low_arch_setup ()
+{
+  target_desc *tdesc
+    = aarch64_create_target_description (0, false);
+
+  static const char *expedite_regs_aarch64[] = { "x29", "sp", "pc", NULL };
+  init_target_desc (tdesc, expedite_regs_aarch64);
+
+  current_process ()->tdesc = tdesc;
+}
+
+/* The singleton target ops object.  */
+
+static netbsd_aarch64_target the_netbsd_aarch64_target;
+
+/* The NetBSD target ops object.  */
+
+netbsd_process_target *the_netbsd_target = &the_netbsd_aarch64_target;