change type of numerical architecture symbols to integer

Following a suggestion in ##5878, this changes the type of the numerical
__CPROVER_architecture_ symbols from int (which is itself
architecture-dependent) to integer (which does not depend on the
architecture).

Closes #5878

Author: kroening

regression/ansi-c/arch_flags_mcpu_bad/object.intel

@@ -113,7 +113,8 @@ static std::string architecture_string(const std::string &value, const char *s)
 template <typename T>
 static std::string architecture_string(T value, const char *s)
 {
-  return std::string("const int " CPROVER_PREFIX "architecture_") +
+  return std::string("const " CPROVER_PREFIX "integer " CPROVER_PREFIX
+                     "architecture_") +
          std::string(s) + "=" + std::to_string(value) + ";\n";
 }