cmd/link: skip TestBuildFortvOS if the SDK is missing

While we're here, move the test from dwarf_test.go to link_test.go;
it doesn't have anything to do with DWARF.

Should fix the macOS builders with only the Xcode command line tools
installed.

Change-Id: Iaaba1b589f4d778705f7b627f78c2b12388e2b3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/168462


TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>