[libunwind] Fix Morello build

Can't use the MTE inline assembly when building for purecap Morello

Author: arichardson

libunwind/src/DwarfInstructions.hpp

@@ -288,7 +288,7 @@ int DwarfInstructions<A, R>::stepWithDwarf(A &addressSpace, pc_t pc,
       // __unw_step_stage2 is not used for cross unwinding, so we use
       // __aarch64__ rather than LIBUNWIND_TARGET_AARCH64 to make sure we are
       // building for AArch64 natively.
-#if defined(__aarch64__)
+#if defined(__aarch64__) && !defined(__CHERI_PURE_CAPABILITY__)
       if (stage2 && cieInfo.mteTaggedFrame) {
         pint_t sp = registers.getSP();
         pint_t p = sp;