sync: use 386 instead of x86-32 to refer to the 32 bit x86 architecture

sync: use 386 instead of x86-32 to refer to the 32 bit x86 architecture

This aligns the naming with GOARCH using 386 as a build target for
this architecture and makes it more easily found when searching
for documentation related to the build target.

Change-Id: I393bb89dd2f71e568124107b13e1b288fbd0c76a
Reviewed-on: https://go-review.googlesource.com/c/go/+/271988
Trust: Martin Möhrmann <moehrmann@google.com>
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>