[嵌入式]Linux kernel ARM64 寄存器tpidr_el1 的用处

存放当前cpu的per_cpu变量的offset值，来自__per_cpu_offset[x]
在计算per_cpu变量对应当前cpu的实际虚拟地址时会用到。

案例(对per_cpu变量overflow_stack的获取)：
[以下代码来自Linux kernel 5.14 rc6]
arch/arm64/kernel/entry.S
/* Switch to the overflow stack */
adr_this_cpu sp, overflow_stack + OVERFLOW_STACK_SIZE, x0

	.macro	get_this_cpu_offset, dst
	mrs	\dst, tpidr_el1
    .endm
	/*
	 * @dst: Result of per_cpu(sym, smp_processor_id()) (can be SP)
	 * @sym: The name of the per-cpu variable
	 * @tmp: scratch register
	 */
	.macro adr_this_cpu, dst, sym, tmp
	adrp	\tmp, \sym
	add	\dst, \tmp, #:lo12:\sym
	get_this_cpu_offset \tmp
	add	\dst, \dst, \tmp
	.endm