unix: add gccgo support

With these changes the package builds, and the tests pass, using gccgo
on amd64 GNU/Linux.  I have not tested other systems but I don't know
why they wouldn't work.

Change-Id: I727365daef55f158657eb89afbfcdbf3334610d1
Reviewed-on: https://go-review.googlesource.com/9842
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

unix/syscall_unix_test.go

@@ -118,6 +118,9 @@ func TestPassFD(t *testing.T) {
 
 	cmd := exec.Command(os.Args[0], "-test.run=^TestPassFD$", "--", tempDir)
 	cmd.Env = []string{"GO_WANT_HELPER_PROCESS=1"}
+	if lp := os.Getenv("LD_LIBRARY_PATH"); lp != "" {
+		cmd.Env = append(cmd.Env, "LD_LIBRARY_PATH="+lp)
+	}
 	cmd.ExtraFiles = []*os.File{writeFile}
 
 	out, err := cmd.CombinedOutput()