[llvm-exegesis] Print signal name when the snippet crashed.

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp

for (auto &CounterName : CounterNames) {
         CrashRecoveryContext CRC;
         CrashRecoveryContext::Enable();
         const bool Crashed = !CRC.RunSafely([this, Counter, ScratchPtr]() {
           Counter->start();
           this->Function(ScratchPtr);
           Counter->stop();
         });
         CrashRecoveryContext::Disable();
-        // FIXME: Better diagnosis.
-        if (Crashed)
-          return make_error<SnippetCrash>("snippet crashed while running");
+        if (Crashed) {
+          std::string Msg = "snippet crashed while running";
+#ifdef LLVM_ON_UNIX
+          // See "Exit Status for Commands":
+          // https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu_chap02.html
+          constexpr const int kSigOffset = 128;
+          if (const char *const SigName = strsignal(CRC.RetCode - kSigOffset)) {
+            Msg += ": ";
+            Msg += SigName;
+          }
+#endif
+          return make_error<SnippetCrash>(std::move(Msg));
+        }
       }
 
       auto ValueOrError = Counter->readOrError(Function.getFunctionBytes());
       if (!ValueOrError)
         return ValueOrError.takeError();
       accumulateCounterValues(ValueOrError.get(), &CounterValues);
     }
     return CounterValues;