illumos gate

Build errors with gcc 9:

    In file included from ../include/Audio.h:31,
                     from AudioCopy.cc:31:
    ../include/AudioError.h: In member function 'AudioError AudioError::operator=(AudioError)':
    ../include/AudioError.h:63:53: error: implicitly-declared 'constexpr AudioError::AudioError(const AudioError&)' is deprecated [-Werror=deprecated-copy]
       63 |      { code = val.code; sys = val.sys; return (*this); }
          |                                                     ^
    ../include/AudioError.h:62:20: note: because 'AudioError' has user-provided 'AudioError AudioError::operator=(AudioError)'
       62 |  inline AudioError operator = (AudioError val) // Assignment
          |                    ^~~~~~~~
    In file included from ../include/AudioStream.h:93,
                     from ../include/AudioBuffer.h:32,
                     from AudioCopy.cc:32:
    ../include/AudioStream_inline.h: In member function 'void AudioStream::setlength(Double)':
    ../include/AudioStream_inline.h:57:11: error: implicitly-declared 'constexpr Double& Double::operator=(const Double&)' is deprecated [-Werror=deprecated-copy]
       57 |  length = len;
          |           ^~~
    In file included from ../include/Audio.h:30,
                     from AudioCopy.cc:31:
    ../include/AudioTypes.h:139:2: note: because 'Double' has user-provided 'Double::Double(const Double&)'
      139 |  Double(const Double &x): val(x.val) { }
          |  ^~~~~~
    In file included from ../include/AudioStream.h:93,
                     from ../include/AudioBuffer.h:32,
                     from AudioCopy.cc:32:
    ../include/AudioStream_inline.h: In member function 'virtual void AudioStream::SetLength(Double)':
    ../include/AudioStream_inline.h:67:12: error: implicitly-declared 'constexpr Double& Double::operator=(const Double&)' is deprecated [-Werror=deprecated-copy]
       67 |   length = len;
          |            ^~~
    In file included from ../include/Audio.h:30,
                     from AudioCopy.cc:31:
    ../include/AudioTypes.h:139:2: note: because 'Double' has user-provided 'Double::Double(const Double&)'
      139 |  Double(const Double &x): val(x.val) { }
          |  ^~~~~~
    cc1plus: all warnings being treated as errors

The same issue as in 12139, we need to provide explicit definitions to prevent removal of implicit versions in future compiler versions.

Testing done: build/install/boot.