Add support for aarch64, powerpc64 and riscv64.

Index: gdb/configure.tgt
--- gdb/configure.tgt.orig
+++ gdb/configure.tgt
@@ -154,6 +154,13 @@ aarch64*-*-linux*)
 			symfile-mem.o linux-record.o"
 	;;
 
+aarch64*-*-openbsd*)
+	# Target: OpenBSD/arm64
+	gdb_target_obs="aarch64-tdep.o aarch64-obsd-tdep.o arch/aarch64-insn.o \
+			obsd-tdep.o solib-svr4.o \
+			symfile-mem.o"
+	;;
+
 alpha*-*-linux*)
 	# Target: Little-endian Alpha running Linux
 	gdb_target_obs="alpha-mdebug-tdep.o alpha-linux-tdep.o \
@@ -494,6 +501,12 @@ powerpc-*-openbsd*)
 	gdb_target_obs="rs6000-tdep.o ppc-sysv-tdep.o ppc-obsd-tdep.o \
 			ravenscar-thread.o ppc-ravenscar-thread.o"
 	;;
+powerpc64-*-openbsd*)
+	# Target: OpenBSD/powerpc64
+	gdb_target_obs="rs6000-tdep.o ppc-sysv-tdep.o ppc64-tdep.o \
+			ppc64-obsd-tdep.o obsd-tdep.o solib-svr4.o \
+			ravenscar-thread.o ppc-ravenscar-thread.o"
+ 	;;
 powerpc-*-aix* | rs6000-*-* | powerpc64-*-aix*)
 	# Target: PowerPC running AIX
 	gdb_target_obs="rs6000-tdep.o rs6000-aix-tdep.o xcoffread.o \
@@ -536,6 +549,12 @@ riscv*-*-linux*)
 	# Target: Linux/RISC-V
 	gdb_target_obs="riscv-linux-tdep.o glibc-tdep.o \
  			linux-tdep.o solib-svr4.o symfile-mem.o linux-record.o"
+	;;
+
+riscv*-*-openbsd*)
+	# Target: OpenBSD/riscv
+	# XXX is that list enough?
+	gdb_target_obs="riscv-obsd-tdep.o"
 	;;
 
 riscv*-*-*)
