runtime: memclrNoHeapPointers optimization for block alignment

runtime: memclrNoHeapPointers optimization for block alignment

goos: linux
goarch: arm64
pkg: runtime
               │  base.log   │               opt.log               │
               │   sec/op    │   sec/op     vs base                │
Memclr/5-4       3.378n ± 2%   3.376n ± 2%        ~ (p=0.128 n=10)
Memclr/16-4      2.749n ± 1%   2.776n ± 2%   +1.00% (p=0.001 n=10)
Memclr/64-4      4.588n ± 2%   4.184n ± 2%   -8.78% (p=0.000 n=10)
Memclr/256-4     8.758n ± 0%   7.103n ± 0%  -18.90% (p=0.000 n=10)
Memclr/4096-4    58.80n ± 0%   57.43n ± 0%   -2.33% (p=0.000 n=10)
Memclr/65536-4   868.7n ± 1%   861.7n ± 1%   -0.80% (p=0.004 n=10)
Memclr/1M-4      23.08µ ± 6%   23.55µ ± 6%        ~ (p=0.739 n=10)
Memclr/4M-4      219.6µ ± 3%   216.1µ ± 2%        ~ (p=0.123 n=10)
Memclr/8M-4      586.1µ ± 1%   586.4µ ± 2%        ~ (p=0.853 n=10)
Memclr/16M-4     1.312m ± 0%   1.311m ± 1%        ~ (p=0.481 n=10)
Memclr/64M-4     5.332m ± 1%   5.681m ± 0%   +6.55% (p=0.000 n=10)
geomean          1.723µ        1.683µ        -2.31%

Change-Id: Icad625065fb1f30b2a4094f3f1e58b4e9b3d841e
Reviewed-on: https://go-review.googlesource.com/c/go/+/616137
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>