cmd/link: on Mach-O, generate LC_UUID by default

cmd/link: on Mach-O, generate LC_UUID by default

On Mach-O, default to "-B gobuildid", so it generates the UUID
based on Go buildid by default.

Fixes #68678.

Cq-Include-Trybots: luci.golang.try:gotip-darwin-amd64_14,gotip-darwin-arm64_13
Change-Id: I6c1a6bcafd8370a13174657e05d7d9620a8d4f12
Reviewed-on: https://go-review.googlesource.com/c/go/+/618598
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Than McIntosh <thanm@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>