runtime: AES maphash scramble 3 times on 386

Issue #43130 shows flaky hash not inbalanced on 386 platform,
which is using AES hashing instead of wyhash.
This CL increase the scramble times to 3 that amd64 using right now.

Fixes #43130

Change-Id: I9d012eda99ff71c13a89448f46fcb9c5e7cec921
Reviewed-on: https://go-review.googlesource.com/c/go/+/384075


Trust: mzh <mzh@golangcn.org>
Run-TryBot: mzh <mzh@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>