With fix for architectures where x<<32 != x.

Original change's description:
> [base] Introduce RoundUpToPowerOfTwo64
>
> And fix RoundUpToPowerOfTwo32 to return 1 for the input 0.
> 0 is no power of two.
> Beside being the correct value, this also avoids a special case in the
> (new) fast path using the number of leading zeros.
>
> R=​jochen@chromium.org, ahaas@chromium.org
>
> Change-Id: I87173495e13b334954bcebbb55724fb666dfa809
> Reviewed-on: https://chromium-review.googlesource.com/488143
> Reviewed-by: Jochen Eisinger <jochen@chromium.org>
> Reviewed-by: Andreas Haas <ahaas@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#44925}

TBR=ahaas@chromium.org,jochen@chromium.org,clemensh@chromium.org,v8-reviews@googlegroups.com

Change-Id: I7b4719d84a419bb7b38e3b5c9d6d183275087ace
Reviewed-on: https://chromium-review.googlesource.com/488981
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44951}