[libunwind][AArch64] Unbreak building with GNU assembler

[xry111]

Summary

GNU assembler mandates armv8.5-a for memtag instructions. Maybe we should remove this restriction in GNU assembler, but let's work around it for current GNU Binutils releases.

[MaskRay]

for current GNU Binutils releases.

It would have been better to include the exact version.

libunwind/src/DwarfInstructions.hpp

#if defined(__aarch64__)
         // interface".
         // [1]:
         // https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#622the-stack
         // [2]:
         // https://github.com/torvalds/linux/blob/1930a6e739c4b4a654a69164dbe39e554d228915/arch/arm64/kernel/signal.c#L718
         p &= ~0xfULL;
         // CFA is the bottom of the current stack frame.
         for (; p < cfa; p += 16) {
-          __asm__ __volatile__(".arch_extension memtag\n"
+          __asm__ __volatile__(".arch armv8.5-a\n"
+                               ".arch_extension memtag\n"
                                "stg %[Ptr], [%[Ptr]]\n"
                                :
                                : [Ptr] "r"(p)
                                : "memory");
         }
       }
 #endif
       // restore registers that DWARF says were saved