Reapply "compiler-rt: Introduce runtime functions for emulated PAC."

This reverts commit 0c0aa56cdcf1fe3970a5f3875db412530512fc07.

This time with the following fixes for buildbot failures:
- Add underscore prefixes to symbol names on Apple platforms.
- Modify the test so that it skips the crash tests on platforms where
  they are not expected to pass:
  - Platforms that implement FEAT_PAuth but not FEAT_FPAC (e.g.
    Apple M1, Cortex-A78C)
  - Platforms where DA key is disabled (e.g. older Linux kernels,
    Linux kernels with PAC disabled, likely Windows)

Original commit message follows:

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: mstorsjo, pawosm-arm, atrosinenko

Reviewed By: atrosinenko

Pull Request: https://github.com/llvm/llvm-project/pull/148094

GitOrigin-RevId: de31584001478d37f6dd645a864d160528979efe
6 files changed