Minimal VM build has new warnings after JDK-8072498 (Multi-thread JNI weak reference processing)

/pool/buildbot/slaves/sobornost/jdkX/build/src/hotspot/share/gc/shared/weakProcessorPhases.cpp: In static member function 'static bool WeakProcessorPhases::is_serial(WeakProcessorPhases::Phase)':
/pool/buildbot/slaves/sobornost/jdkX/build/src/hotspot/share/gc/shared/weakProcessorPhases.cpp:62:46: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]
   return (index(phase) - serial_phase_start) < serial_phase_count;
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

This because serial_phase_count ends up being "0" with Minimal VM, because JVMTI and JFR are disabled:

  enum Phase {
    // Serial phases.
    JVMTI_ONLY(jvmti COMMA)
    JFR_ONLY(jfr COMMA)

    // OopStorage phases.
    jni,
    vm
  };

  static const uint serial_phase_start = 0;
  static const uint serial_phase_count = jni;

This can be worked around with -Wno-type-limits. We probably want to tag the usage in weakProcessorPhases.cpp:62 as safe one, instead of disabling that compiler warning wholesale.