runtime: pad work.full and pad.empty to avoid false sharing

runtime: pad work.full and pad.empty to avoid false sharing

With the Garbage collector (GC), we observed a false-sharing between
work.full and work.empty. (referenced most from runtime.gcDrain and
runtime.getempty)

This false-sharing becomes worse and impact performance on multi core
system. On Intel Xeon 8480+ and default GC setting(GC=100), we can
observed top HITM>4% (by perf c2c)caused by it.

After resolveed this false-sharing issue, we can get performance 8%~9.7%
improved. Verify workloads:

DeathStarBench/hotelReservation: 9.7% of RPS improved
https://github.com/delimitrou/DeathStarBench/tree/master/hotelReservation

gRPC-go/benchmark: 8% of RPS improved
https://github.com/grpc/grpc-go/tree/master/benchmark

gRPC-go/benchmark 9 iterations' data with master branch:
	master	        w/ fs opt.
	208862.4	246390.9
	221680.0	266019.3
	223886.9	248789.7
	212169.3	257837.8
	219922.4	234331.8
	197401.7	261627.7
	214562.4	255429.7
	214328.5	237087.8
	229443.2	230591.3

max	229443.2	266019.3	116%
med	214562.4	248789.7	116%
avg	215806.3	248678.5	115%

Change-Id: Ib386de021cd2dbb802a107f487556d848ba9212d
Reviewed-on: https://go-review.googlesource.com/c/go/+/496915
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>