[RFC/RFT] CFI: Add support for gcc CFI in aarch64

[Dan Li]

+ Cc: linux-hardening at vger.kernel.org
On 12/18, Dan Li wrote:
> Based on Sami's patch[1], this patch makes the corresponding kernel
> configuration of CFI available when compiling the kernel with the gcc[2].
> 
> The code after enabling cfi is as follows:
> 
> int (*p)(void);
> int func (int)
> {
> 	p();
> }
> 
> __cfi_func:
>         .4byte 0x439d3502
> func:
>         ......
>         adrp    x0, p
>         add     x0, x0, :lo12:p
>         mov     w1, 23592
>         movk    w1, 0x4601, lsl 16
>         cmp     w0, w1
>         beq     .L2
>         ......
>         bl      cfi_check_failed
> .L2:
>         blr     x19
>         ret
> 
> In the compiler part[4], there are some differences from Sami's
> implementation[3], mainly including: