cmd/go: skip link_syso tests in short mode

cmd/go: skip link_syso tests in short mode

These tests invoke the system C compiler and linker.
Skipping them saves a little over half a second of time in short mode.

Updates #54423.

Change-Id: I3e8aa7b53c0c91f7d1e001ec2cd5f7b4954de52d
Reviewed-on: https://go-review.googlesource.com/c/go/+/425206
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>