cmd/compile: intrinsify math/bits/ReverseBytes{32|64} for 386
The BSWAPL instruction is supported in i486 and newer.
https://github.com/golang/go/wiki/MinimumRequirements#386 says we
support "All Pentium MMX or later". The Pentium is also referred to as
i586, so that we are safe with these instructions.
Change-Id: I6dea1f9d864a45bb07c8f8f35a81cfe16cca216c
Reviewed-on: https://go-review.googlesource.com/c/go/+/465515
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org> Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Keith Randall <khr@google.com>