[lld] [PAC][lld] Use braa instr in PAC PLT sequence with valid PAuth core info (PR #113945)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Assume PAC instructions being supported with PAuth core info different from (0,0). Given that, autia1716; br x17 can be replaced with braa x17, x16; nop.

It is worth explaining what braa is.

https://github.com/llvm/llvm-project/pull/113945