cmd/go: enable fuzz testing for FreeBSD

cmd/go: enable fuzz testing for FreeBSD

Add "freebsd" to GOOS for which sys.FuzzSupported() returns true
and add freebsd to the build tags to fuzz test source.

Fixes #46554
Change-Id: I5f695ecc8f09c0ab4279ced23b4715b788fcade0
Reviewed-on: https://go-review.googlesource.com/c/go/+/377855
Trust: Bryan Mills <bcmills@google.com>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>