[lld][MachO] Add ARM64e pointer authentication linking support

[oskarwirga]

This PR implements ARM64e linking support in lld's Mach-O backend. This is a continuation of work discussed in #79543 and I included @BertalanD's suggestion to keep the Relocation object size small, thanks! This PR is part of several I have planned, but contains everything needed to link arm64e Mach-Os with LLD.

What this PR adds

• Chained fixup pointer structures: Adds bitfield definitions covering both legacy (16-bit ordinal) and USERLAND24 (24-bit ordinal) formats.
• AUTH relocation parsing: Extends lld's Relocation struct with auth metadata (diversity, key, address-diversity) parsed from ARM64_RELOC_AUTHENTICATED_POINTER relocations. Auth fields are packed in a union with the 64-bit addend to keep sizeof(Relocation) at 24 bytes.
• Authenticated stubs
• Set default CPU_SUBTYPE_ARM64E with ptrauth ABI version 0, I got that from Apple's LD
• Dual GOT: Separate __auth_got section for signed function pointers alongside the existing unsigned __got. Stubs load from __auth_got; address-taken data references use __got.
• Symbol routing: Stub creation targets __auth_got; personality pointers in unwind info route to __auth_got; ObjC message send fast stubs use __auth_got for objc_msgSend lookup with braa authentication
• Chained fixup encoding: Encodes auth-rebase, auth-bind, plain-rebase, and plain-bind entries. Automatically selects DYLD_CHAINED_PTR_ARM64E_USERLAND24 format based on deployment target (macOS 12.0+, iOS 15.0+, watchOS 8.0+, tvOS 15.0+)

Testing

• arm64e-stubs.s — Verifies authenticated stub code sequence
• arm64e-auth-got.s — Verifies dual GOT (__auth_got + __got) creation and routing
• arm64e-auth-reloc.s — Verifies ARM64_RELOC_AUTHENTICATED_POINTER parsing and chained fixup encoding
• arm64e-auth-data.s — Verifies auth metadata (key, diversity, addrDiv) flows through the Relocation union into chained fixups
• arm64e-chained-fixups.s — Verifies chained fixup format selection and encoding
• arm64e-reject-mixed.s — Verifies arm64/arm64e object mixing rejection
• arm64e-no-fixup-chains.s — Verifies -no_fixup_chains is rejected for arm64e
• End-to-end tested with iOS/macOS apps

AI Disclosure

I used Claude heavily throughout development, primarily to iterate through linker errors. All generated code was manually reviewed and tested on ARM64e hardware. I feel very confident in these changes being functionally correct because I tested on hardware extensively, architecturally they may be lacking!