[libunwind][arm64e] Mach-O Support unwinding PAuth_LR frames (#202772)

Under PAuth_LR, compact unwind entries marked UNWIND_ARM64_MODE_FRAME_PAUTH_LR are signed with the PC of the pacibsppc at the start of the range. The compiler may emit those entries with UNWIND_IS_NOT_FUNCTION_START when the pacibsppc is not the first instruction in the function; the unwinder must scan backwards past such entries to find the actual function start (and its LSDA pointer).

When unwinding a PAuth_LR frame, authenticate and re-sign using setIPPAuthLR(), which calls __builtin_ptrauth_auth_with_pc_and_resign when available, falling back to inline asm (autib171615 + pacib) for compilers that lack the builtin.

To reduce the size of Compact Unwind entries for shrinkwrapped frames, new offset bits encode the distance in multiples of 4 bytes from the range start to the pacibsppc. The compiler emits paired FRAMELESS/FRAMELESS+PAUTH_LR entries that the linker merges into a single FRAME_PAUTH_LR | (offset/4) << 12 encoding.

rdar://161907302&169746432&171118419

NOKEYCHECK=True
GitOrigin-RevId: 8e34b2cbb06a14fd9b38625eccf7fdb482ea5ea3