arm64: Add support for STACKLEAK gcc plugin

This adds support for the STACKLEAK gcc plugin to arm64 by implementing
stackleak_check_alloca(), based heavily on the x86 version, and adding the
two helpers used by the stackleak common code: current_top_of_stack() and
on_thread_stack(). The stack erasure calls are made at syscall returns.
Additionally, this disables the plugin in hypervisor and EFI stub code,
which are out of scope for the protection.

Acked-by: Alexander Popov <alex.popov@linux.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/entry.S

@@ -902,6 +902,9 @@ ret_to_user:
 	cbnz	x2, work_pending
 finish_ret_to_user:
 	enable_step_tsk x1, x2
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	bl	stackleak_erase
+#endif
 	kernel_exit 0
 ENDPROC(ret_to_user)