les: rework arm64's undefined workaround

[gballet]

The previous fix #21960 converted the float to an intermediate signed int, before attempting the uint conversion. Although this works, this doesn't guarantee that other architectures will work the same.

[holiman]

simpler:

	if base >= 0{
		e.Base += uint64(base)
	}else if uint64(-base) <= e.base{
		// The conversion from negative float64 to
		// uint64 is undefined in golang, and doesn't
		// work with ARMv8. More details at:
		// https://github.com/golang/go/issues/43047
		e.Base -= uint64(-base)
	}
	return amount

... ?

[holiman]

Ah nevermind...