runtime: use correct truncated constants for float conversion

There is a range of numbers lower than 0x7fff_ffff_ffff_ffff which
cannot be represented by a 64 bit float. We set that to the correct
limit beyond which conversions can happen properly.

It appears that the negative bound check can indeed by correctly handled
by I64TruncF64S. But we use the same limit for consistency.

Fixes #38839

Change-Id: Ib783a22cb331fba7e6955459f41c67f9ceb53461
Reviewed-on: https://go-review.googlesource.com/c/go/+/231837


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>