Move JVMFlag origins into a new enum JVMFlagOrigin

Many JVM function take an JVMFlag::Flags parameter to indicate the origin of the flag -- i.e., "who is setting this flag". E.g., in arguments.hpp:

  static bool parse_argument(const char* arg, JVMFlag::Flags origin);

However, JVMFlag::Flags contains many other bits that are unrelated to the origin. We should add a new enum JVMFlagOrigin that has only the valid values for the origin. This makes it possible to do more type-safety checks at C++ compilation time.