hash/maphash: add purego implementation

hash/maphash: add purego implementation

Fixes #47342

Change-Id: I9c3b36de3e7c85d3e7aff2f84abc78a1b382008c
Reviewed-on: https://go-review.googlesource.com/c/go/+/468795
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>