Reuses the existing logic for BigInt.parseInt, adapted slightly
to allow octal and binary radix prefixes (and to support parsing
of a raw character buffer, rather than a v8::internal::String).

Bug: v8:6791
Change-Id: I41904b2204721eac452e0765fa9ff0ab26ee343b
Reviewed-on: https://chromium-review.googlesource.com/711334
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48560}