sync: align poolLocal to CPU cache line size

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>

src/sync/pool.go

@@ -54,11 +54,18 @@ type Pool struct {
 }
 
 // Local per-P Pool appendix.
-type poolLocal struct {
+type poolLocalInternal struct {
 	private interface{}   // Can be used only by the respective P.
 	shared  []interface{} // Can be used by any P.
 	Mutex                 // Protects shared.
-	pad     [128]byte     // Prevents false sharing.
+}
+
+type poolLocal struct {
+	poolLocalInternal
+
+	// Prevents false sharing on widespread platforms with
+	// 128 mod (cache line size) = 0 .
+	pad [128 - unsafe.Sizeof(poolLocalInternal{})%128]byte
 }
 
 // from runtime