base-linux: 64-bit ARM support

This patch adds support for running Genode/Linux on the AARCH64
architecture.

- The kernel-agnostic startup code (crt0) had to be extended to
  capture the initial stack pointer, which the Linux kernel uses
  to pass the process environment. This is in line with the
  existing startup code for x86_32 and x86_64.

- The link order of the host libraries linked to lx_hybrid
  programs had to be adjusted such that libgcc appears at last
  because the other libraries depend on symbols provided by
  libgcc.

- When using AARCH64 Linux as host, one can execute run scripts
  via 'make run/<script> KERNEL=linux BOARD=linux' now.

Issue #4136

repos/base/lib/mk/spec/arm_64/startup.inc

@@ -0,0 +1,3 @@
+include $(BASE_DIR)/lib/mk/startup.inc
+
+vpath crt0.s $(call select_from_repositories,src/lib/startup/spec/arm_64)

repos/base/src/lib/startup/spec/arm_64/crt0.s

@@ -5,7 +5,7 @@
  */
 
 /*
- * Copyright (C) 2019 Genode Labs GmbH
+ * Copyright (C) 2019-2021 Genode Labs GmbH
  *
  * This file is part of the Genode OS framework, which is distributed
  * under the terms of the GNU Affero General Public License version 3.
@@ -24,6 +24,12 @@
 	.global _start_initial_stack
 	_start_initial_stack:
 
+        /* save initial SP value, used to pass the environment on base-linux */
+	adrp x4, :got:__initial_sp
+	ldr  x4, [x4, #:got_lo12:__initial_sp]
+	mov  x1, sp
+	str  x1, [x4]
+
 	/*
 	 * Install initial temporary environment that is replaced later by the
 	 * environment that init_main_thread creates.
@@ -53,6 +59,10 @@
  *********************************/
 
 .bss
+	/* initial value of the SP register */
+	.global __initial_sp
+	__initial_sp:
+	.space 8
 
 	/* stack of the temporary initial environment */
 	.p2align 8