cmd/go: adjust test with tricky ld flags to not run on Darwin
normally this would not run on darwin anyway, but if there
happens to be a "pkg-config" binary, then it will. Darwin's
clang/linker does not have this flag.
Change-Id: I61929cf193c95b420441187beb8b237fa1037807
Reviewed-on: https://go-review.googlesource.com/c/go/+/616376
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Sam Thanawalla <samthanawalla@google.com> Reviewed-by: Michael Matloob <matloob@golang.org>