Currently, Number.prototype.toString(radix) often fails to produce the
least significant bit for doubles near zero. For example, for the
minimum double, 5e-324, toString(2) produces "0". This means that a
user cannot reliably get the exact binary or hexdecimal value of a
double from JavaScript using toString.

This patch makes a slight amendment to the DoubleToRadixCString
function, so that doubles where the gap to the next double is 5e-324
(i.e. doubles less than 2**-1021), are represented exactly in binary and
other power-of-two bases, and close to exactly otherwise. It results
in Number.prototype.toString producing the correct binary value for all
doubles.

R=jkummerow@chromium.org, mathias@chromium.org, yangguo@chromium.org

Bug: v8:9294
Change-Id: I71506149b7c4c0eac8c38675a1ee15fb4f36f9ef
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1631601
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61925}