Make poolLocal size multiple of 128, so it aligns to CPU cache line
on the most common architectures.

This also has the following benefits:

- It may help compiler substituting integer multiplication
  by bit shift inside indexLocal.
- It shrinks poolLocal size from 176 bytes to 128 bytes on amd64,
  so now it fits two cache lines (or a single cache line on certain
  Intel CPUs - see https://software.intel.com/en-us/articles/optimizing-application-performance-on-intel-coret-microarchitecture-using-hardware-implemented-prefetchers).

No measurable performance changes on linux/amd64 and linux/386.

Change-Id: I11df0f064718a662e77a85d88b8a15a8919f25e9
Reviewed-on: https://go-review.googlesource.com/40918Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>