Support for Apple arm64e and arm64e.x1 (Pointer Authentication & Checked Pointer Arithmetic) targets

[jpoumailloux]

Hi folks,

We would like to upstream support for Apple's arm64e and arm64e.x1 ABIs on 64-bit Darwin ARM (ios and macos) in two small, self-contained PRs:

1. arm64e target feature & Pointer Authentication (PAC) codegen:

   • Adds Target::ARM64e (arm64e), valid on 64-bit arm with ios or macos (implying armv83a and arm_fp16, and treated as an ABI-matching feature in get_runtime_compatible_target).
   • Sets llvm::Triple::AArch64SubArch_arm64e on ios and macos, defaults iOS mcpu_target() to apple-a12, and adds the Darwin arm64e ptrauth-* / aarch64-jump-table-hardening function attributes.
   • Signs non-callee function references via llvm::ConstantPtrAuth (IA key 0, discriminator 0, matching Darwin userspace arm64e), attaches "ptrauth"(i32 0, i64 0) operand bundles to indirect calls across AOT and JIT modules, and signs JIT-resolved function pointers via llvm::orc::ExecutorAddr::toPtr on ptrauth_calls hosts.

2. arm64e_x1 target feature (FEAT_PAuth_LR + FEAT_CPA2):

   • Adds Target::ARM64eX1 (arm64e_x1, implying arm64e, armv87a, arm_fp16, and arm_dot_prod), using apple-m5 (currently the newest Apple CPU model in LLVM) as mcpu_target() and enabling +pauth-lr (branch-protection-pauth-lr) and +cpa (-aarch64-use-featcpa-codegen=true).
   • Emits the versioned CPU_SUBTYPE_ARM64E_X1 (0x8000000c) Mach-O cpusubtype when writing Darwin arm64e object files with +cpa.

Please let us know if the target naming (arm64e / arm64e_x1) or overall design looks good, or if you prefer any adjustments!