runtime/internal/sys: remove Bswap{32|64} assembly for 386

runtime/internal/sys: remove Bswap{32|64} assembly for 386

CL 465515 instrinsify these functions, so that we can remove them.

Change-Id: I81c9eb62ecf56d5a292dc5aced542b36b53847dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/465516
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>