Fix fatal SEHException on Arm calling RDTSC

Arm does not have the x86 Timestamp Counter register or read timestamp
counter instruction. Add an additional architecture check to prevent
the code path when running on Arm.

Author: jonorossi

src/NetMQ/Core/Utils/OpCode.cs

@@ -12,13 +12,21 @@ internal static class Opcode
 
         public static bool Open()
         {
+#if NETSTANDARD1_1_OR_GREATER || NET471_OR_GREATER
+            if (RuntimeInformation.ProcessArchitecture != Architecture.X86 &&
+                RuntimeInformation.ProcessArchitecture != Architecture.X64)
+            {
+                return false; // RDTSC instruction not supported
+            }
+#endif
+
             var p = (int)Environment.OSVersion.Platform;
 
             byte[] rdtscCode = IntPtr.Size == 4 ? RDTSC_32 : RDTSC_64;
 
             s_size = (ulong)(rdtscCode.Length);
 
-            if ((p == 4) || (p == 128))
+            if ((p == 4) || (p == 128)) // Unix || Mono on Unix
             {
                 // Unix
                 if (IsARMArchitecture()) return false;