Add Audio Support

[ellamosi]

Add support for audio following the documentation within the GB Development Wiki: https://gbdev.gg8.se/wiki/articles/Gameboy_sound_hardware

What's supported

• [x] Square 1 Channel with Frequency Sweep
• [x] Square 2 Channel
• [x] Wave Channel
• [x] Noise Channel
• [x] Blargg's DMG Audio Tests, except the ones that rely on mid instruction timings:
  • 09-wave read while on
  • 10-wave trigger while on
  • 12-wave write while on
• [x] Documentation

What's not supported (left for another iteration)

• Zombie mode
• Blargg's DMG Audio Tests requiring mid instruction timings
• Non DMG specific behavior
• Prehistorik Man's Intro might destroy ears (Zombie mode related probably)
• Smooth Player Will probably require some changes to make it work, properly emulating the analog high pass filter instead of taking shortcuts for instance.

Additional Notes

• Updates GNAT compiler URL
• Fixes HALT flag not being initialized (was breaking O2 optimized builds)
• Minor non audio related cleanups