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>

src/cmd/go/go_test.go

@@ -1505,13 +1505,16 @@ func main() {
 	tg.setenv("PKG_CONFIG_PATH", tg.path("."))
 	tg.run("run", tg.path("foo.go"))
 
-	// test for ldflags
-	tg.tempFile("bar.pc", `
+	if runtime.GOOS != "darwin" { // darwin doesn't like these ldflags
+		// test for ldflags
+		tg.tempFile("bar.pc", `
 Name: bar
 Description: The bar library
 Version: 1.0.0
 Libs: -Wl,-rpath=/path\ with\ spaces/bin
 `)
+	}
+
 	tg.tempFile("bar.go", `package main
 /*
 #cgo pkg-config: bar