compiler-rt: Introduce runtime functions for emulated PAC.
The emulated PAC runtime functions emulate the ARMv8.3a pointer
authentication instructions and are intended for use in heterogeneous
testing environments. For more information, see the associated RFC:
https://discourse.llvm.org/t/rfc-emulated-pac/85557
Reviewers: llvm-beanz, petrhosek
Pull Request: https://github.com/llvm/llvm-project/pull/133530
