[dev.fuzz] cmd/go: skip fuzz tests on unsupported platforms

[dev.fuzz] cmd/go: skip fuzz tests on unsupported platforms

For now, fuzzing is only supported on Darwin, Linux, and Windows. We
haven't testing shared memory communication between coordinator and
worker processes on other platforms. Currently, fuzz.CoordinateFuzzing
and RunFuzzWorker both panic with "unimplemented" messages.

Unfortunately, these may be masked by runtime.Goexit calls that
override panics, so some builders fail with

    test executed panic(nil) or runtime.Goexit

Change-Id: Ic7bd692461958626dd5c5bb31198aec86387bbd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/290810
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>