les: cosmetic rewrite of the arm64 float bug workaround (ethereum#21960)

* les: revert arm float bug workaround to check go 1.15

* add traces to reproduce outside travis

* simpler workaround

les/utils/expiredvalue.go

@@ -88,8 +88,9 @@ func (e *ExpiredValue) Add(amount int64, logOffset Fixed64) int64 {
 	if base >= 0 || uint64(-base) <= e.Base {
 		// This is a temporary fix to circumvent a golang
 		// uint conversion issue on arm64, which needs to
-		// be investigated further. FIXME
-		e.Base = uint64(int64(e.Base) + int64(base))
+		// be investigated further. More details at:
+		// https://github.com/golang/go/issues/43047
+		e.Base += uint64(int64(base))
 		return amount
 	}
 	net := int64(-float64(e.Base) / factor)