[PAC] Implement jump table hardening

[asl]

Downstream patch: ahmedbougacha@038ab43

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Anton Korobeynikov (asl)

Downstream patch: https://github.com/ahmedbougacha/llvm-project/commit/038ab43b4dd28fba7e732aa4fa0f36c51508c722