Gekko: Migrate register structs over to Common::BitField

[lioncash]

Provides the same behavior, while also preventing undesirable type conversions and promotions on bitfield members in certain cases.

e.g.

union Thing
{
  u32 hex = 0;

  struct
  {
    u32 value_1 : 5;
    u32 padding : 27;
  };
};

void SomeFunction(const Thing& thing)
{
  // Assume the encoding stores some value - 1, so we add to get the full value
  const u32 data = thing.value_1 + 1;
}

The intuitive (and completely expected most of the time) reading of this would be to think that value_1 undergoes no type promotions or conversions, since everything is unsigned, however, this line:

  const u32 data = thing.value_1 + 1;

Actually results in value_1 being converted to an int, incremented, and then assigned to data, implicitly converting it back to a u32

UGQR was already using this as well, so this keeps the register layouts consistent as well.

[leoetlino]

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Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @lioncash)