[clang] [ARM] Using cp15 while mtp =auto and arch is arm_arch6k and support thumb2 (PR #130027)

[Vladi Krapp via cfe-commits]

VladiKrapp-Arm wrote:

@Zhenhang1213 :
Code looks ok. 
Are all edge cases covered by the lit test now? 

Could you please adjust the commit message to reflect the actual changes? 

https://github.com/llvm/llvm-project/pull/130027