fixed stack setup when using SMP. (#7978)

libcpu/aarch64/cortex-a/entry_point.S

@@ -331,7 +331,11 @@ after_mmu_enable_cpux:
     b rt_hw_secondary_cpu_bsp_start
 #endif
 
+#ifndef RT_CPUS_NR
+#define RT_CPUS_NR 1
+#endif
+
 .align 12
 .el_stack:
-.space (8192)
-.el_stack_top:
+.space (8192 * RT_CPUS_NR)
+.el_stack_top: