// Note that these semantics differ from x86: the carry flag has the opposite
// sense on a subtraction!
//
//	On amd64, C=1 represents a borrow, e.g. SBB on amd64 does x - y - C.
//	On arm64, C=0 represents a borrow, e.g. SBC on arm64 does x - y - ^C.
//	 (because it does x + ^y + C).
//
// See https://en.wikipedia.org/wiki/Carry_flag#Vs._borrow_flag
type flagConstant uint8