[PAC][libunwind] Unify signed return address handling logic

[kovdan01]

#143230 introduces libunwind hardening which could be used with pauth-enabled ABIs (such as arm64e for Apple and pauthtest for Linux). This includes handling of signed return addresses. Both mentioned ABIs implicitly enable -fptrauth-returns flag, which sets __has_feature(ptrauth_returns) to true and triggers RA signing in codegen via setting "ptrauth-returns" attribute for functions in IR.

The existing logic in libunwind already handles signed RA - such handling was introduced previously to support signed RA appearing when -mbranch-protection=pac-ret is used:

llvm-project/libunwind/src/DwarfInstructions.hpp

Lines 303 to 344 in 3cb2174

	#if defined(_LIBUNWIND_TARGET_AARCH64)
	// If the target is aarch64 then the return address may have been signed
	// using the v8.3 pointer authentication extensions. The original
	// return address needs to be authenticated before the return address is
	// restored. autia1716 is used instead of autia as autia1716 assembles
	// to a NOP on pre-v8.3a architectures.
	if ((R::getArch() == REGISTERS_ARM64) &&
	isReturnAddressSigned(addressSpace, registers, cfa, prolog) &&
	returnAddress != 0) {
	#if !defined(_LIBUNWIND_IS_NATIVE_ONLY)
	return UNW_ECROSSRASIGNING;
	#else
	register unsigned long long x17 __asm("x17") = returnAddress;
	register unsigned long long x16 __asm("x16") = cfa;
	// We use the hint versions of the authentication instructions below to
	// ensure they're assembled by the compiler even for targets with no
	// FEAT_PAuth/FEAT_PAuth_LR support.
	if (isReturnAddressSignedWithPC(addressSpace, registers, cfa, prolog)) {
	register unsigned long long x15 __asm("x15") =
	prolog.ptrAuthDiversifier;
	if (cieInfo.addressesSignedWithBKey) {
	asm("hint 0x27\n\t" // pacm
	"hint 0xe"
	: "+r"(x17)
	: "r"(x16), "r"(x15)); // autib1716
	} else {
	asm("hint 0x27\n\t" // pacm
	"hint 0xc"
	: "+r"(x17)
	: "r"(x16), "r"(x15)); // autia1716
	}
	} else {
	if (cieInfo.addressesSignedWithBKey)
	asm("hint 0xe" : "+r"(x17) : "r"(x16)); // autib1716
	else
	asm("hint 0xc" : "+r"(x17) : "r"(x16)); // autia1716
	}
	returnAddress = x17;
	#endif
	}
	#endif

Note that existing implementation in libunwind supports different RA sign state in different stack frames. For example, we can build libunwind itself w/o pac-ret enabled, and build user code with pac-ret enabled, and unwinding would work fine since we look at RA sign state in each individual frame.

After #143230, signed RA handling looks as follows:

1. If libunwind is built with pauth-enabled ABIs such as arm64e and pauthtest (which imply __has_feature(ptrauth_calls) && __has_feature(ptrauth_returns), we skip old signed RA handling logic and use signed RA handling logic from [runtimes][PAC] Harden unwinding when possible #143230.
2. Otherwise, we use old signed RA handling logic.

Actual codegen is the same for -fptrauth-returns and -mbranch-protection=pac-ret, we just have different ways enabling that (see also #143230 (comment)).

We should unify logic in libunwind for these two cases as well.

[kovdan01]

Tagging @asl @ojhunt @atrosinenko