armv8: fix gd after relocation

Commit 21f4486faa ("armv8: update gd after relocate") sets x18
without checking the return value of spl_relocate_stack_gd().

Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: York Sun <york.sun@nxp.com>
CC: Kever Yang <kever.yang@rock-chips.com>
CC: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

arch/arm/lib/crt0_64.S

@@ -120,8 +120,9 @@ relocation_return:
 #endif /* !CONFIG_SPL_BUILD */
 #if defined(CONFIG_SPL_BUILD)
 	bl	spl_relocate_stack_gd           /* may return NULL */
-	/* set up gd here, outside any C code */
-	mov	x18, x0
+	/* set up gd here, outside any C code, if new stack is returned */
+	cmp	x0, #0
+	csel	x18, x0, x18, ne
 	/*
 	 * Perform 'sp = (x0 != NULL) ? x0 : sp' while working
 	 * around the constraint that conditional moves can not