libunwind: Allow untagged pc under benchmark ABI

Under the benchmark ABI, when making a function call that never returns, the link register might contain an untagged capability generated by a previous function return (Recall that returning under the benchmark ABI involves clearing the LSB of the address, clearing the tag at the same time).

libunwind/src/AddressSpace.hpp

@@ -247,7 +247,8 @@ class _LIBUNWIND_HIDDEN LocalAddressSpace {
     }
     bool isNull() const { return value == nullptr; }
     bool isValid() const {
-#ifdef __CHERI_PURE_CAPABILITY__
+#if defined(__CHERI_PURE_CAPABILITY__) &&                                      \
+    !defined(__ARM_MORELLO_PURECAP_BENCHMARK_ABI)
       return __builtin_cheri_tag_get(value);
 #else
       return !isNull();