runtime: don't assume that 0.25 * 100 is representable as int

For the gofrontend, it's not; there are some trailing one bits,
which is permitted by the spec.

Change-Id: I58f5a5347068bb488306ec8e73da7c59376f1ae1
Reviewed-on: https://go-review.googlesource.com/c/go/+/383635


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>