[llvm] [AArch64][BTI] Mark EH landing pads as jump targets (PR #149680)

[Benjamin Herrenschmidt via llvm-commits]

ozbenh wrote:

Same with llvm20 on Fedora:
Before:
```
.../...
  400b70:	97ffff8a 	bl	400998 <__cxa_throw at plt>
  400b74:	14000021 	b	400bf8 <_fini>
  400b78:	f9000be0 	str	x0, [sp, #16]
  400b7c:	2a0103e8 	mov	w8, w1
.../...
```
After:
``` 
  400b70:	97ffff8a 	bl	400998 <__cxa_throw at plt>
  400b74:	14000023 	b	400c00 <_fini>
  400b78:	d503249f 	bti	j
  400b7c:	f9000be0 	str	x0, [sp, #16]
  400b80:	2a0103e8 	mov	w8, w1
```

But it's still crashing. I've confirmed that libgcc branches past the `bti` instruction:
```
=> 0xfffff7c57350 <_Unwind_RaiseException+260>:	br	x6
(gdb) p/x $x6
$2 = 0x400b7c
(gdb) x/i $x6
   0x400b7c <main()+80>:	str	x0, [sp, #16]
(gdb) x/i $x6-8
   0x400b74 <main()+72>:	b	0x400c00 <_fini>
(gdb) 
   0x400b78 <main()+76>:	bti	j
(gdb) 
   0x400b7c <main()+80>:	str	x0, [sp, #16]
```


https://github.com/llvm/llvm-project/pull/149680