Comment out support for mtag extension in libunwind (#85431)

Works around build breaks with some compiler versions.

Fixes dotnet/source-build#3424

Author: jkotas

src/native/external/llvm-libunwind/src/DwarfInstructions.hpp

@@ -211,7 +211,7 @@ int DwarfInstructions<A, R>::stepWithDwarf(A &addressSpace, pint_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 0 // defined(__aarch64__)
       if (stage2 && cieInfo.mteTaggedFrame) {
         pint_t sp = registers.getSP();
         pint_t p = sp;